Add simulation of white light images with ray tracing

[johan12345]

The original IDL implementation of GCS includes a feature to generate simulated white light coronagraph images using a ray tracing algorithm based on the given GCS geometry and an assumed electron density distribution.

Someone asked me whether this functionality is included in this Python version, and so far it is not.

I am no longer actively working on this code as I left the field last year, but let's keep the issue open in case someone decides to implement it and contribute it to the library.