unintuitive colorbar defaults

[thenaomig]

This is showing up in warming_levels.ipynb, run with all of the defaults. The colorbar is a diverging one even though all of the data is positive, which results in a lot of blue pixels (implying colder temperatures) when every pixel is in-fact warmer.

[vicford]

This is based on the default behavior in _get_cmap. Will modify this function to determine the min/max range first to establish which colormap + consideration of moisture variables which should have "blues" represent more moisture as following standard meteorological convention.