Create external scripts to break up lib/plotting_functions into organized helper methods

[justin-richling]

This PR will organize helper methods that currently live in lib/plotting_functions.py with the intent of bringing any ADF-wide utilities (non plotting related) to an external script lib/adf_untils.py so methods such as validating dimensions like zm_validate_dims, or averaging schemes like seasonal_mean, or any other non-plotting helpers like mask_land_or_ocean, etc would now fall under this script.

A second external script lib/plotting_utils.py is created for helper methods related to plotting such as prep_contour_plot and get_central_longitude, etc. This script would be called in plotting_functions.py so that plotting_functions.py can be specific to only plotting schemes for diagnostic plotting scripts.

Breaking these methods up into external scripts will not only organize helper functions better, but it will eliminate roughly 1000 lines from plotting_functions.py, specifically ones that are not related to plotting.

Additionally, this PR will create a warnings function in lib/adf_utils.py

#Format warning messages:
def my_formatwarning(msg, *args, **kwargs):
    """Issue `msg` as warning."""
    return str(msg) + '\n'

that is defined in most scripts currently so it can be centralized and called in scripts via adf_utils:

import warnings  # use to warn user about missing files.
import adf_utils as utils
warnings.formatwarning = utils.my_formatwarning

I'm open to also making a plotting directory in lib to organize plotting scripts since we will probably want to extend color maps to NCL soon thanks to the work of @brianpm

[nusbaume]

Thanks for the great cleanup @justin-richling! I have some change requests, but hopefully they will all be easy to handle (it is mostly about commented-out lines of code).

[justin-richling]

Hi @nusbaume, it looks like almost all of these requested changes are resolved; I had some erroneous code that got committed in to this branch. My apologies for the extra work, but could you double check the plotting_functions.py and plotting_utils.py again when you get a chance? Thanks!

[nusbaume]

Thanks for removing the erroneous code @justin-richling! I believe that has resolved all of my concerns except for the one request related to amwg_table.py. Once that is resolved this PR will be good to go (at least for me)!

[nusbaume]

Everything looks great to me now. Thanks again!