Simplify/Secure the mexHierarchical_fact function by reviewing the translation...

Simplify/Secure the mexHierarchical_fact function by reviewing the translation of matlab params struct to C++ Faust::Params equivalent.

- Simplifying by refactoring utility functions to mx2Faust.h/c(pp) (mxArray2FaustParams() added), deleting the forest of if-else in testCoherence().
- Securing by avoiding to rely on number indices to identify fields but rather rely on constants (enum MAT_FIELD added for that purpose, with conversion functions from and to string).
- This update will be useful to write matlab wrapper to FactHierarchicalFactF(G)FT.

Related secondary change:
- Removing useless M (matrix) argument in matfaust.FaustFactory.fact_hierarchical() in init. of params struct.

It's not perfect: mxArray2FaustParams() could be refactored.

This modification was discussed before for 32790a99 next to issue #66.