Iron name change and equations set/problem specification array

- Prefix is now cmfe_
- C header is now opencmiss/iron.h
- Python module is opencmiss.iron, and opencmiss is a namespace package so we can have opencmiss.zinc separate
- Problems are now identified by a specification array, rather than class, type and subtype. The `cmfe_Problem_CreateStart` routine takes a specification array rather than having the separate `SpecificationSet` routine.
- Equations sets also identified by a specification array

I guess we should send an email to the mailing list letting people know about this change before merging it? There's a script at `utils/update_fortran_program_prefix.py` for updating Fortran programs to use the new prefix and specification arrays.