port amake Fortran code to generate rate grid for area source on the fly

Fortran codes amake, amakeshear, and amakeshearMP are used to generate gridded rates. The simplest, amake, distributes a uniform rate to grid nodes within an area source polygon, scaling only by latitude and normalizing to the total (node) area. Codes amakeshear and amakeshearMP also apply gaussian smoothing and/or account for shear zones. nshmp-lib should generate these rate grids on the fly.

For initial implementation, generate scaled rates at grid nodes within an area source defined by a GeoJSON feature.

Also want to consider uniformely spaced grid in km that would then have the same rate at each point.