Magnitude scaling relation consistency

[pmpowers-usgs]

Migrated from usgs/nshm-cous-2014#4 and usgs/nshmp-haz-fortran#19

In the 2014 model, a somerville MSR was used for point-source distance corrections but WC94 continued to be used for fixed strike RLME grid sources. Moreover, several magnitude scaling relations are used to broaden the epistemic uncertainty of Casacadia unsegmented rupture magnitudes, but then Youngs/Geomatrix is use to compute length for floating (segmented) ruptures.