RMerosivity modifications

Created by: rbcarvin

I made several changes to RMerosivity. It now has a few more terms to define when calling the function. While there are a few things that could be improved, this version has been tested against known 'answers', and it runs reliably. The data must have all rows with zero rainfall removed before running this function, and the user must determine the appropriate value to use in the timeInterval term -- I would like to improve this function to automate those steps in the future.