Improve Python documentation generated with sphinx

[Thoemi09]

Summary

The goal of this issue is to improve the generated Python documentation. Right now, sphinx simply generates a list of all functions and classes and dumps them into two big tables without any proper documentation. This is in contrast to the doxygen documentation, where we use groups to organize functions into logical units.

Check list

• Use Python modules to model the top-level group structure from doxygen
• Use common markdown files to document the various groups --> avoid to parse the doxygen group doc strings
• Improve the brief function description of dispatched C++ functions by using the first brief doc string encountered for an overload
• Improve class documentation by using Python types instead of C++ types
• For dispatched C++ constructors use the same function documentation as for other dispatched functions

This is work in progress and the check list should and will be updated accordingly.