Avoid error from invalid floating point operation

If theta_grid_knobs::equilibrium_option isn't eik, then neither surfarea nor dvdrhon are set, resulting in an invalid floating point operation error. grhoavg is already calculated, so just use that. It will still be undefined if eik isn't used, but at least we won't be dividing by NaNs.

Another solution would be to only set these variables if eik is used

Update to ensure that theta_grid provides these values for all geometry types.