rmax_geo is not recorded

[bprather]

In new polarization imaging there's been renewed interest in changing the outer radius for treating plasma & Faraday rotation effects, rmax_geo.

The default value in ipole is 100M, but this is sometimes set to 50 or lower to reflect only the portion of the simulation in infall equilibrium. The difference is relevant in near edge-on images of SgrA*.

Currently this is neither logged to stdout nor to image files, and this would be easy to change.