Improve R-factor calculation

[akrherz]

Reviewing the GloRESatE paper and @ryanpmcg 's Chapter 16 paper, I have come to the conclusion that my present R-factor calculator that uses DEP's WEPP-formatted breakpoint files is lacking...

Things to address:

• Presently, the calculation has a daily closure to match the storage of the computed value within the daily data structure. This is probably a fine caveat, but we should also have a routine that more robustly considers storms crossing midnight and operates at a yearly closure.
• There's presently no storm exclusion (event over >12.7mm), again, another life choice that likely should be controlled by a method parameter
• There is no "dry" logic to separate storm events (<1.27mm over 6h). This should always be implemented.
• The DEP breakpoint data is 5 or 2 minute intervals, which is aggregated to a regularized 30 minute interval. There's no right answer here, but this should be controllable and transparent to the user.