Update M7-8 interface ruptures to be more distributed

Currently the narrow and wide M7-8 floating rupture implementation uses the NSHM_SUB_GEOMAT_LENGTH length scaling relation to model rupture length. However, rupture width is always the full down-dip width of the interface. This puts more M7-8 rate over the entire width of the interface than would occur if we could support the use of magnitude-area relations and float ruptures down dip as well. But this is difficult because the grids for interface ruptures are non-uniform. In PRVI, we're starting to use grid centered planar (quad) rupture in OpenSHA and support for this will soon be added to nshamp-haz. In the meantime, we favor using pure point sources for the M7-8 interface ruptures (i.e. model M7-8 as gridded seismicity). However instead of the spatial pdf used for the interface gridded seismicity model, we'll use a spatial pdf that implies a uniform rate over the entire arc, adjusted for latitude.