Update HPX specs and sample files

[woodmd]

This PR includes a number of revisions to the HPX spec and sample files:

• Combines BANDS/EBOUNDS/ENERGIES tables such that EBOUNDS and ENERGIES are now treated as special cases of the BANDS table. The BANDSHDU header keyword defines the extension name of the BANDS table. If BANDSHDU is undefined the BANDS table should be read from EBOUNDS or ENERGIES. This makes the format backwards-compatible with existing model and counts cubes that use the convention of an EBOUNDS or ENERGIES table.
• Updates SPARSE format specification to include an explicit band index column (CHANNEL). This should address the concerns raised by @eacharles (see HEALPix Formats #80) about keeping the slice indexing in the same HDU as the map data. However this does moderately increase the file size (up to 25% when VALUE and PIX are both 32-bit).
• Samples files now include sparse maps generated with both fixed and energy-dependent NSIDE.
• Other miscellaneous cleanup and fixes in the spec.