Fix delegation of unhandled stellar file event to a script

Previously, an unhandled stellar file directive is transferred to
a Squirrel function ::CoordSys.readFile[0], which itself looks confusing.
It also lead errors by unclear reason.

Now we define the scripted function simple and straightforward way;
it's named stellarReadFile.
Move hard-coded rstation processing logic for stellar file to init.nut

The Resource Station is one of a few classes that can be defined in the
stellar structure definition files.  The logic is hard-coded into
CoordSys::readFile(), but it's best implemented in a script for easy
customization.

The hard-coded logic is commented out anyway.
Separated RStation-draw.cpp from RStation.cpp

Also added Doxygen comments on top of each files.
Make RStation be rendered visible

The class have not had a model to show, because it has been stuck with
precompiled .bin files which are obsolete.