Improvements to temporal metadata interpretation

[tomvothecoder]

From 4/12 meeting notes:

1. Explore CF Convention attribute indicating what the coordinate points to in relation to time bounds (e.g., midpoint of time bounds or at of month)
2. Support for interpretation of CF Convention cell_methods attribute to determine the method for generating time bounds
   • Handling E3SM time coordinates (always at the end of the time interval) -> if attr exists then can set end_of_month flag to True, but might not exist
   • Can use center_times flag
   • CMOR seems to handle centering time coordinates or maybe apart of the conversion code to CMORize (check e3sm_to_cmip)