This is the initial commit of the GCPy plot modules refactoring.  We
have split up gcpy/plot.py into:

gcpy/plot/core.py
gcpy/plot/six_plot.py
gcpy/plot/single_panel.py
gcpy/plot/compare_single_level.py
gcpy/plot/compare_zonal_mean.py

Some issues remain but we just wanted to stash code here before
moving on.  These issues will be resolved in subsequent commits.

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

gcpy/plot/compare_single_level.py
- Break up strings across multiple lines to avoid "line-too-long"
  warning from Pylint
- Trimmed trailing whitespace

gcpy/plot/compare_zonal_mean.py
- Convert target_index from numpy.int64 to int, to avoid a Pylint
  "Sequence index" warning.  (List indices must be int but not
   numpy.int32 or numpy.int64)
- Break up strings across multiple lines to avoid "line-too-long"
  warning from Pylint
- Now import six_panel_subplot names from gcpy.plot.core (so no
  need to call core.six_panel_subplot_names).
- Removed if __name__ == "__main__" block

gcpy/plot/core.py
- Removed unused import statement (as per Pylint)
- Removed "self" from call to super().__init in MidpointLogNorm class
- Removed if __name__ == "__main__" block

gcpy/plot/single_panel.py
- Now import normalize_colors from gcpy.plot.core (and replace the
  core.normalize_colors call with normalize_colors)
- Removed if __name__ == "__main__" block

gcpy/plot/six_plot.py
- Remove unused import statements
- Remove debug print statements

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

gcpy/plot/six_plot.py
- Split the computation of vmin, vmax for absdiff and ratio plots
  into separate functions called by compute_vmin_vmax_for_plot
- Make sure that all functions return a value (to avoid an
  "inconsistent-return-statements" warning by Pylint
- Updated comments
- Trimmed trailing whitespace

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

gcpy/benchmark_funcs.py
- Now import constants individually from gcpy.constants
- Import compare_single_level from gcpy.plot.compare_single_level
- Import compare_zonal_mean from gcpy.plot.compare_zonal_mean
- Remove unneeded "warning_format" variable

gcpy/examples/diagnostics/compare_diags.py
- Remove unused import for xarray
- Import functions individually from gcpy.util
- Import constants individually from gcpy.constants
- Import compare_single_level from gcpy.plot.compare_single_level
- Import compare_zonal_mean from gcpy.plot.compare_zonal_mean
- Now use dataset_reader to return the proper reader
- Fix exception handling

gcpy/examples/plotting/plot_comparisons.py
- Import compare_single_level from gcpy.plot.compare_single_level
- Import compare_zonal_mean from gcpy.plot.compare_zonal_mean

gcpy/examples/plotting/plot_single_panel.py
- Import single_panel from gcpy.plot.single_panel

gcpy/examples/timeseries/mda8_o3_timeseries
- Update plot styles for Matplolib 3.7.2

gcpy/gcpy_plot_style
- Removed, this is now in the plot folder

gcpy/plot.py
- Removed, this has now been split up to other modules in the plot folder

gcpy/gcpy
gcpy/gcpy.examples
- Remove these extraneous files

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

gcpy/benchmark/benchmark_slurm.sh
- Now activate the "gcpy_env" environment, which is the same name
  in the default environment.yml file (and in the documentation)

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

gcpy/six_plot.py
- In routine "colorbar_for_small_data_range", now use
  pos = [vmin, vmin+vrange*0.25, vmin+vrange*0.5, vmin+vrange*0.75, vmax]
  which should place the intermediate tickpoints symmetrically.

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

This merge brings the feature/plot-subdir up-to-date with the dev
branch (which now has the fixes for regridding that were added
by PR #266).

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

docs/environment_files/environment.yml
- Add the "tk" library to the GCPy python environment YAML file

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

gcpy/examples/diagnostics/compare_diags.py
- Fixed typo "thed" -> "the"

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

CHANGELOG.md
- Now state that the 'tk' package has been added to the
  docs/environment_files/environment.yml file.

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