Create separate code to perform comparison analysis of results

[monocongo]

Compare results of new code against operational results from NCEI climate divisions with the following variables computed:

For each division:

1. differences by calendar month, with line plots (expected, actual, diffs) (use matplotlib)
2. differences per month, with diff maps (use either WCT or Fenimore's process)
3. RMSE, by calendar month, and total
4. % with change of sign, by calendar month, and total
5. % with positive bias, % with negative bias

Write the code in a fashion that makes it easy to refactor out a base class for reuse with code that will do this for grids, so we can do a similar comparison using WRCC/WWDT PRISM datasets.

[monocongo]

Create nClimDiv datasets with all currently supported indices computed from inputs provided by NCEI monitoring branch, for both methods of calculating PET. From these datasets compute statistics (RMSE, %delta sign, etc.) as well as line graphs and histograms of differences.

[monocongo]

created branch 'comparison_analysis' for this issue

[monocongo]

Add a command line option, associated function parameters, and functionality to use either the original NCDC method or Thornthwaite's method for PET calculation, in order to determine the effect of this variable on the overall PDSI (and SPEI) results.