Merge PR geoschem#253 (Fix broken code in file_regrid.py; updated doc)

This merge brings PR geoschem#253 (Fixed broken regridding code in file_regrid.py;
Updated GCPy regridding documentation, by @yantosca) into the GCPy
1.4.0 development stream.

We have fixed several issues in the current gcpy/file_regrid.py script,
so that LL -> CS/SG, LL -> LL, CS/SG -> CS/SG, and CS/SG -> LL regridding
now work.  Furthermore, we have refactored the code in file_regrid.py
for clarity.

We have updated the Regridding.rst file and have added some extra
examples using file_regrid.py.  We have also verified that all examples
using file_regrid.py and regrid_restart_file.py work as advertised.

This will not affect benchmark code as the plotting routines
compare_zonal_mean() and compare_single_level() use the routine
regrid_comparison_data from gcpy/regrid.py.  Perhaps we can resolve
the issue reported in geoschem#167 with the knowledge thus gained in preparing
this PR.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

CHANGELOG.md

@@ -44,10 +44,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Added coding suggestions made by `pylint` where possible
 - Abstracted and never-nested code from `six_plot` into functions (in `plot.py`)
 - Added `main()` routine to `gcpy/file_regrid.py`; Also added updates suggested by Pylint
-
-### Removed
-- Removed `gchp_is_pre_13_1` arguments & code from benchmarking routines
-- Removed `is_pre_13_1` tags from `*_benchmark.yml` config files
+- Fixed broken regridding code in `gcpy/file_regrid.py`; also refactored for clarity
+- Rewrote `Regridding.rst` page; Confirmed that regridding examples work properly
 
 ### Fixed
 - Generalized test for GCHP or GCClassic restart file in `regrid_restart_file.py`