[mkstratos]

This enables plots of snow properties (snow cover, snow storage) from ELM.

To do so, a few updates were needed:

• Single dataset plotting
• Dual-axis (left and right) seasonal cycle plots
  • Configure in the field configuration section with ac_axis: [0 / 1] for left or right
• New "snow" module

And a few updates happened along the way:

• Configurable figure DPI (img_dpi in the general config, default is 90 dpi)
• Configurable figure output extension (img_extn in general config, default is PNG)
• Better error handling: the whole analysis doesn't crash if one file is missing! (Depends on LIVVkit >= v3.4.0)
• Adds documentation to all livvext.common and livvext.util methods
• Lays framework for accepting non Jan-Dec climatologies