RMerosivity depends on RMintensity-calculated I30

[rbcarvin]

RMerosivity formula uses the 30-minute intensity, right now the user is a allowed to specify which intensities they want returned, I30 needs to always be included.

[rbcarvin]

@limnoliver I found the dependency RMerosivity has on RMintensity. The df output from RMintensity can be named anything, but must include that I30 for use in the erosivity formula. Do you think we should hard code the I30 into RMintensity? or explore recalculating it within RMerosivity?