Update to isd_generate.py to allow for a NAIF radius override

Based on this RFC, we want to change our Mars pipeline to use a sphere. https://astrodiscuss.usgs.gov/t/appl-rfc-use-iau-sphere-for-hirise-dtm-projects-in-socet-set/428

Currently ALE uses the Mars elliptical defaults from the NAIF kernels. This update allows the user to override the kernels and set a spherical radius for both semimajor and semiminor in the ISD.

This is not only needed for Mars. For triaxial bodies, the IAU recommends a best-fit sphere should also be set for map products. References (DOI) are listed in the updated help for the current IAU values.