Metadata symbology enhancements

[ceholden]

Related to #77:

1. Enabler: specify qualitative/sequential colormap alongside the "friendly name" and metadata value/observation and handle this choice when initializing metadata item colors
2. Cleanup: alter metadata handling to use one variable (dict) that stores properties rather than multiple variables storing this information separately:
   • name (str)
   • values (np.ndarray)
   • colormap (enum{qualitative, sequential})
   • include_table (bool)
3. Finalize: implement month calculation as standard metadata item in drivers