Implement `build` module for custom reference files

[mstubb]

The build module should take sets of V and J (and C) sequences and make the appropriate resources.

Things to think about for the build module.

1. C sequences are currently hard-coded. These should be moved out into resource files and loaded as appropriate for the necessary species.
2. iNKT seqs are similarly hard-coded. Should also split these out and make them optional if the expected sequences are not known.
3. Should change resources directory structure so that it goes something like:

species ----> imgt_seqs
        |
        |---> synthetic_genomes
        |
        |---> igblast_dbs
        |
        |---> const_seqs
        |
        |---> invariant_cells

Tracer can then check that a particular species exists before loading the appropriate data.

The invariant_cells folder could contain sequences for any kind of invariant cell so that it could also deal with MAIT cells and anything else.