Allow cath-ssap to restrict regions in PDB_DSSP mode

[tonyelewis]

At present, a command like this:

cath-ssap --prot-src-files PDB_DSSP 3h5q 4giu --align-regions 'D[3h5qA01]-2-67:A' --align-regions 'D[4giuA01]24-90:A'

...generates an error with key message Cannot yet restrict to regions with this combination of input files.

---

Full error message:

Whilst running program cath-ssap (via a program_exception_wrapper with typeid: "N4cath30ssap_program_exception_wrapperE"), caught a boost::exception:
/ftp/software/cath-tools/source/structure/protein/protein.cpp(462): Throw in function void cath::restrict_to_regions(cath::protein&, const region_vec_opt&)
Dynamic exception type: boost::exception_detail::clone_impl<cath::common::not_implemented_exception>
std::exception::what: Cannot yet restrict to regions with this combination of input files