recalculate 10 year / 20 year historical norms

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.

Start by reviewing issue #89 and inspecting the modis-terra and modis_terra directories to locate the code and data required to regenerate historical norms. Document the regeneration process, then rerun the calculations for the 10-year and 20-year norms; done means the procedure is documented and both recalculations have completed.

Written by the indexing model from the issue text.