Store chromosome names as HDF5 enum

[nvictus]

Rather than fixed-length strings, the chroms/name dataset should be stored as HDF5 enums, just as the bins/chrom dataset is.

[nvictus]

Because of the header size limit, this is a bad idea. The only other thing to consider is whether to use variable length strings instead and whether to use UTF-8 encoding or just store bytes and assume ASCII as now.

[nvictus]

Conclusion: ASCII for arrays of strings, Variable-length UTF-8 for single strings.