From Patricia: data_prep.py update description regarding time ranges

[benhudson85]

From Patricia:

The start of the range for future is historic end range +1. The future range start year is not actually used. Might be a good idea to add some guidance/explanation on this. If this is not a correct interpretation please advise.

## Year ranges
historic_range = [1990,2011]
rollback_range = [1990,2011]
future_range = [2012,2070]