standalone flip fails when "reduced.chrom.sizes" provided, should go lexicographical

[sergpolly]

e.g. , when pairtools flip --chroms-path hg38.reduced.chrom.sizes p1.pairs, where hg38.reduced.chrom.sizes does not have un-assembled contigs - it fails when encounters unrecognized name - e.g. KeyError: 'chrUn_KI270438v1'

in the same situation pairtools parse behaves differently - it flips what it can according to hg38.reduced.chrom.sizes and flips lexicographically everything else

no big deal - but we'd rather have it documented - to fix later

[agalitsyna]

Thanks, resolved by: #117
@sergpolly , it would be great if you can check this toy example with unannotated chromosome: https://github.com/open2c/pairtools/blob/7480fb500a3d4db2c43ea9d98f4c65764c68a1e5/tests/data/mock.4flip.pairs

If that's what you expect, then let's close it!