Integrated whatsnew for v3.1 release (rc0) (SciTools#4285)

* Replace interim 3.0.x whatsnews with single v3.1.rst, with concrete version and date.

* Mark as unreleased, fix links.

* Rearrange entries tagged 'pre-3.1', and recategorise SciTools#4176 change.

* Moved bugfix notes from v3.1 to v3.0 whatsnew page.

* Fixed broken pull-request refs.

* Fix link to PyKE.

Test wrangling vs v3.0.x (SciTools#4249)

* test changes

* add awol init

* rename system test cases

* fix PartialDateTime tests

* fix typo

* add whatsnew

Update 3.0.3.rst

Update release date in `3.0.3.rst`

add release date to v3.0.2 whatsnew (SciTools#4160)

Logging fixes (SciTools#4106)

Promote unknown units to dimensionless in aux factories (SciTools#3965)

* promote unknown to dimensionless units in aux factories

* patch aux factories to promote unknown to dimensionless units for formula terms

* add whatnew PR for entry

update github ci checks image (SciTools#3957)

Add whatsnew announcement (SciTools#3900)

Prep for 2.4 release. (SciTools#3654)

Define Iris 2.4 (SciTools#3638)

* Bump version to 2.4.0rc0.

* unpin mpl (SciTools#3468)

* Merge pull request SciTools#3301 from bayliffe/fastpercentilemethod_mask_test

Analysis percentile method - update applicability test for fast_percentile_method

* Have Travis test with iris-grib, remove problem tests (SciTools#3469)

* Have Travis test with iris-grib, remove problem tests

* mock GribInternalError correctly

* Update license headers

* account for changes in handling of grib message defaults

* Test against the latest version of python-eccodes

* Moved irir-grib skip to iris.tests

* Merge pull request SciTools#2608 from cpelley/PICKLEABLE_FORMATS

TEST: Extends SciTools#2569 to unpickle

* _regrid_area_weighted_array: Move axes creation over which weights are calculated to before loop (SciTools#3519)

* Purge iris.experimental.regrid np<1.7 support (SciTools#3539)

* Add NameConstraint with relaxed name loading (SciTools#3463)

* _regrid_area_weighted_array:  move indices variable nearer to use (SciTools#3564)

* _regrid_area_weighted_array: Tweak variable order to near other use in code (SciTools#3571)

* Fix problems with export and echo command. (SciTools#3577)

* Pushdocs fix2 (SciTools#3580)

* Revert to single-line command for doctr invocation.

* Added script comment, partly to force Github respin.

* Bracketed six.moves and __future__ imports.

* Fixes required due to the release of iris-grib v0.15.0 (SciTools#3582)

* Fix python-eccodes pin in travis (SciTools#3593)

* PI-2472: Optimise the area weighted regridding routine (SciTools#3598)

* PI-2472: Tweak area weighting regrid move xdim and ydim axes (SciTools#3594)

* _regrid_area_weighted_array: Set axis order to y_dim, x_dim last dimensions

* _regrid_area_weighted_array: Extra tests for axes ordering

* PI-2472: Tweak area weighting regrid enforce xdim ydim (SciTools#3595)

* _regrid_area_weighted_array: Set axis order to y_dim, x_dim last dimensions

* _regrid_area_weighted_array: Extra tests for axes ordering

* _regrid_area_weighted_array: Ensure x_dim and y_dim

* PI-2472: Tweak area weighting regrid move averaging out of loop (SciTools#3596)

* _regrid_area_weighted_array: Refactor weights and move averaging outside loop

* Pin pillow to make graphics tests work again. (SciTools#3630)

* PI-2472: Area weighted regridding (SciTools#3623)

* The Area-weights routine is refactored into the "__prepare" and "__perform" structure, in-line with our other regridding methods.
* The area-weights are now computed in the "__prepare", so are calculated once.
* The information required for checking the regridding weights and target grid info are now cached on the "regridder" object.
* This is inline with the general use already described in the documentation.

* pep8 conformance.

* pep8 compliance.

* Allow some 'key=None' args in Geostationary creation. (SciTools#3628)

LGTM

* Allow some 'key=None' args in Geostationary creation.

* Integration test loading netcdf geostationary without offset properties.

* pep8 conformance.

* pep8 conformance.

* pep8 conformance.

* test_NameConstraint get mock from iris.tests.

* Remove use of super() in _constraints.py for Py2 compatibility.

* Updated license headers.

* Updated iris-grib reference in extensions.txt.

* Py2 support for iris-grib in Travis.

* Updates for auto docs for Iris 2.4 release.

* What's new entry to unpinning mpl.

* Edited Py2 support for iris-grib in Travis.

* Renamed whatsnew contributions folder for v2.4.

* Hacked tests.integration.test_grib2 to avoid import error from iris-grib version < 0.15.

* Only test grib with python 3.

* Compiled v2.4 whatsnew.

Co-authored-by: Martin Yeo <[email protected]>
Co-authored-by: Bill Little <[email protected]>
Co-authored-by: stephenworsley <[email protected]>
Co-authored-by: abooton <[email protected]>
Co-authored-by: lbdreyer <[email protected]>
Co-authored-by: Emma Hogan <[email protected]>