Change numbering and naming standards with convert()

[xapple]

Citing @jrougemo

When converting a Track to say bed format, we could have the option of choosing a different numbering/naming convention, for example use "ensembl" convention (1-based, all-inclusive numbering) with possibly different chromosome names (to be fetched from genrep).

[xapple]

Two new methods exist to convert between the different numbering conventions.

from bbcflib import track
with track.load('tracks/rp_genes.bed') as t:
   t.ucsc_to_ensembl()
   t.ensembl_to_ucsc()