Improved plotting of shap

[gabrieltseng]

So far:

• Allow polished variable names to be plotted in the shap plots
• Be better about plotting the timestamp of the input data, instead of just the index

The results of the latest iteration of plots can be found at https://github.com/esowc/ml_drought/blob/analysis/plotting/notebooks/draft/15_gt_ealstm.ipynb