Grid spacing configuration option to override spacing in map.geojson

It would be helpful to be able to set the grid spacing as a configuration option, either in the AWS configuration submitted to the AWS web app or in the hazard configuration. This would allow reusing a single map.geojson file (even one of the standard map files, e.g. nshmp-haz/etc/nshm/nshm-conus.geojson) for different test runs. This would make it easier to store map files in gitlab/github and reference from the AWS config file rather than uploading a file for each individual calculation

The grid spacing in the map.geojson file could be overridden by the AWS app when downloading the target map file (e.g. a jq one-liner), or by nshmp-haz when generating the list of sites, though it may be good for the sites-file.geojson to reflect the actual sites run.