recalculate 10 year / 20 year historical norms

[franTarkenton]

This has not been run since the data was originally provided by the hatfield team.

Task:

• find and document the code that is required to regenerate the historical norm data.
• re-run the calculations for 10y / 20y historical norms

relates: #89 - the code that calculates the historical norms likely is dependent on either the modis-terra or the modis_terra directories, and expects all the data to exist in one of those folders.