Rename file monom.hh to monomial.hh, and directory 'monom' to 'monomial'

This was decided on the 2014 dev meeting in Berlin.  'monom' is a Germanism.
Add backward-compatibility header 'monom.hh'

All it does is include the new header 'monomial.hh', which does what the
old 'monom.hh' header does, only under a new name.
Canonicalize the single-inclusion macro