Update version in Project.toml

Tagged version v0.3.1 did not update Project.toml, so bump the
patch version again, this time with matching version in the project.

Fix tshift! for Julia v0.7+; closes #6

- Use broadcast assignment to set parts of the trace to 0 when
  shifting without wrapping.
- Remove the use of a global SAC verbose variable and use
  a keyword argument instead.
- Add tests for tshift[!].

Drop v0.6 support; add Project.toml

- Drop support for Julia v0.6 and require v0.7+.
- Convert the package into a Pkg3 project with a Project.toml.
  This should allow other packages to `add` or `dev` it.

Improve compile time regression on v0.7+

On v0.7+, SAC spends up to a minute compiling the SACtr function
for reading Vector{Int8}, which used to be @eval'ed to avoid
typing out all the fields.

Instead, loop over fields and use setfield!.  This reduces
compile times back to v0.6 behaviour and doesn't slow down the
code at all.

Add AbstractFFTs and FFTW to REQUIRE

Although these aren't needed on v0.6, add them to REQUIRE
for compatibility with Pkg on v0.7+.  This decreases
import time considerably.

Add methods to correct functions for FFTs on v0.7

Fix flip_component[!] (or flip[!])

Fix a bug where the new component inclination was wrong.
The old approach had the wrong convention, whilst SAC
uses inclinations measured downwards from 0 vertically
upward.

Add tests for flip_component! as well.

Replace deprecated atan2 uses; bump Compat; don't import Base.fft on …

…v0.7-

Fix missing StatsBase in REQUIRE

Check pairs of traces have same delta when performing linear operatio…

…ns; fix braodcasting depwarn on v0.7 in tests

- Check the delta is the same for two traces when adding (etc.)
  them together.  Before we only checked they were the same length
  and had the same start time.
- Add check for the above.
- On v0.7, filling an array on the LHS with a scalar requires
  broascasting the assignment (i.e., need a[1:2] .= 3).