Update documentation

[timlichtenberg]

• Create basic description of code structure
• Describe main features and current functionality
• Update incrementally while adding new features

[lsoucasse]

Is this issue still valid @timlichtenberg? Or should we update it to include the points we discussed recently (having specific install instructions for Kapteyn cluster / pointing to documentation of individuals submodules).

[nichollsh]

This is something which should be continually performed as issues come and go. I am closing this issue as there are no specific targets associated with it, and the documentation is now in a much better state compared to when this issue was first created.