Fix typo represenation/representation in a number of files

[balducci]

hi there

this PR fixes a trivial typo represenation/representation in a number of sources, which was causing compile failures like:

    molecules-container.cc:5481:1: error: no declaration matches 'void molecules_container_t::export_molecular_represenation_as_gltf(int, const std::string&, const std::string&, const std::string&, int, const std::string&)'
     5481 | molecules_container_t::export_molecular_represenation_as_gltf(int imol, const std::string &atom_selection_cid,
          | ^~~~~~~~~~~~~~~~~~~~~
    molecules-container.cc:5481:1: note: no functions named 'void molecules_container_t::export_molecular_represenation_as_gltf(int, const std::string&, const std::string&, const std::string&, int, const std::string&)'
    In file included from molecules-container.cc:32:
    molecules-container.hh:41:7: note: 'class molecules_container_t' defined here
       41 | class molecules_container_t {
          |       ^~~~~~~~~~~~~~~~~~~~~
    make[2]: *** [Makefile:801: molecules-container.lo] Error 1

[pemsley]

Ha - I was trying to fix up the documentation extracted from the header - I didn't try to compile before commiting those changes. Shame we couldn't get together last week.