Allow user to define seasonal boundaries for climatology_average

[hCraker]

Describe the feature you'd like added to this project
Currently climatology_average does not allow the user to select the seasonal boundaries. Seasonal climatologies are only calculated for the meteorological seasons, which are defined as follows:

• Winter: December, January, February
• Spring: March, April, May
• Summer: June, July, August
• Fall: September, October, November

This functionality should be added to the climatology module since it is available in NCL's month_to_season family of functions.

Link to original NCL functionality, if applicable
month_to_season
month_to_season12
month_to_seasonN

Describe the solution you'd like
This could be done either by modifying climatology_average or by implementing one-to-one copies of NCL's month_to_season12 and month_to_seasonN in Python.

climatology_average could be modified to take a parameter defining the first season of interest. This can be done using the NCL convention of three letter strings to represent the months (i.e. JFM for January, February, March as the first season). The function can take this and infer the other three seasons. Then climatologies for all four seasons can be calculated. This would not exactly replicate functionality in NCL's month_to_season functions and may be too specialized of a solution. Input from users is needed.

[hCraker]

I'm raising this to high priority because a user requested this during the ESDS event