Extra sanity checking: when multiple inputs are given, check that contig lengths match?

[fedarko]

Sort of a sequel to #32.

We have contig lengths easily available for BCF files (since we enforce this information is given in the header for each contig) and for BAM files (using the .lengths property of a pysam.AlignmentFile object). It'd be nice to add these checks at the start of each command (should be able to abstract the actual work to a utility function).

I don't imagine that this will come up in practice much, but I'm sure it'll happen sooner or later -- since failing to account for this will cause a smorgasbord of silly errors.

• [x] Add a utility function for testing that BAM/BCF files' lengths match FASTA lengths
• [x] Modify phasing_utils.load_triplet() to use both these checks
• [x] Modify call to use the BAM-FASTA check
• [x] Modify fdr estimate to use the BCF-FASTA check
• [ ] Modify matrix compute to use the BAM-FASTA check (when it's implemented...)
• [x] Modify align to check that FASTA lengths match lengths in the GFA file

[fedarko]

Closing, since everything that can use these checks now does!