Create general purpose GES model

[rmward61]

The GES model has been developed to be specific to the geometry of the space. The next step is to develop a version which can be easily adapted to different geometries by setting geometry parameters. This is reasonably straightforward.
Also required is a solar radiation model - this has been implemented in the MATLAB version of the code so just needs adding to the python version

[mhauru]

For the current status of the GES model, see #250