osteosarc 0.13.1

Faster commands (#78): three to eight times faster once a snapshot has been used. On the current snapshot:

command	before	now
reads, on a cached result	7.9 s	2.8 s
files	6.1 s	1.6 s
samples T1_tumor	7.5 s	1.8 s
variants	1.8 s	0.2 s
--version	0.5 s	0.2 s

• A snapshot's file, header and variant catalogues are saved in the cache the first time, and loaded after.
  • Each user keeps their own copies, which can hold only osteosarc's catalogue classes.
  • A loaded copy warns of stale corrections as a built one does.
• Files and corrections are found through indexes, not by scanning.
• pandas is imported only to download a file.

Install: python -m pip install --upgrade osteosarc. No API changes.