Correct singularity boundary point limit

Logical `has_lon_jumps` is used to decide if to use a simple average
(fast) or a shorter-distance average (slow) for longitude. The switch
controls `periodicity` input argument for __mean??_lon functions.

Currently, longitude is judged to have jumps if
1. Two adjacent points have difference larger than 180 degrees.
2. The North Pole is within the domain but not at the boundaries.

This commit should help speed up most of 2D interpolations.

Allow GMesh to work with other projections like polar stereographic.
`lonlat` argument changes the meaning of self.lon and self.lat to
x, y in any projections.

is_geo_coord is added as an optional argument in instantiation. If true,
 (default), lon and lat are of their literal meanings, which are
geographic coordinates. If false, lon and lat are arbitrary x/y
coordinates.

When is_geo_coord is False, all interpolations options related to lon
and lat are turned off, including refine_in_3d, pole singularity and
periodical longitude.

The simple flag enables refine and regrid with arbitrary map projection
like polar stereographic.