ENH: Add a function for forming a composite function from a list
ENH: Improve the automatic message/moments conversion system

Nodes require that their parents' moments are exactly of the correct
types. However, often the moments can be compatible and a simple
conversion can be written. For instance, Gaussian Markov chain moments
to Gaussian moments. These conversions are performed automatically.
However, previously there had to be one conversion between exactly those
moments. Now, the conversion is more robust because it may use several
conversions to achieve the requested total conversion. In addition,
inheritance in moments is handled properly. Moment classes just need to
add conversions to the _converters dictionary attribute in order to
enable conversions. All the complex total conversion is computed then
automatically.
TST: Add unit tests for the new moments conversion system
MAINT: Make clean conversions of Notebooks to RST and Python
DOC: Add empty sections for sketching the documentation structure