Feature request: better modeling of a pysam sequence dictionary

[nh13]

Rough sketch of functionality:

• class that takes the pysam list of dictionaries reader.header["SQ"] and stores something similar to fgbio's SequenceMetadata
• method that takes a FASTA and returns the path to the sequence dictionary (simple as Path(fasta).with_suffix(".dict")?)
• method that takes a FASTA and returns the sequence dictionary class

[nh13]

Working on this...