Allen catalogue (#1071)

Add a new catalogue comprising optimised version of some of the mechanisms commonly used in models of the Allen Cell Types Database, eg http://celltypes.brain-map.org/mouse/experiment/electrophysiology/323535089.
This made required extending modcc to some new NMODL features, as well as to expose some optimisations.

In addition clean up the default catalogue by moving test mechanisms to their proper place in the unit tests.