Support compound CHARSETs

[josephwb]

Currently things are as:

'data_file' is a simple 'vanilla' Nexus file containing sequences and defined CHARSETs.
  - PLEASE NOTE! Only simple CHARSETs are currently supported.
    - e.g. contiguous (X-Y) or interval (e.g. codon: X-Y\3) data are fine.
    - CHARSET referencing is NOT allowed at present (but will be!)

Need to allow more complex partitions, e.g.:

CHARSET p10 = 66259-66387, 101052-104953, 104954-105897, 105898-106034, 106035-106848, 106849-111795;

Will require a change to how partition-specific sites are stored.