generate-catalogue is gone

[thorstenhater]

This is prep for meson in addition to simplifying (ie reducing) the amount of scripting
we have. In the long run, I'd like to remove BuildModules.cmake too, but this might
be superseded by meson and this PR is self-contained.

Changes

• modcc accepts now a list of NMODL files
• likewise, modcc is now able to spit out a catalogue.cpp file
• remove the generate-catalogue script, simplify a-b-c
• remove some options from modcc we never use
• No longer allow external modcc

TODO

• Re-enable raw CXX files when building catalogues.

[brenthuisman]

TODO looks done, correct?

[thorstenhater]

✅

[brenthuisman]

I think doc/dev/extending_catalogues.rst can use an update. Looks like it never mentioned this script (only in a comment).

[thorstenhater]

Done, and the overall 'API' is much simpler again