Add predictor artifact fetch and listing

[iskandr]

Summary

• add mhctools fetch and Python fetch() for predictor artifacts
• delegate MHCflurry presentation and affinity downloads to MHCflurry's native manager
• add mhctools ls / JSON inventory with manager, version, path, and fetchability
• inventory Calis parameters and Pepsickle package weights already shipped with installed packages
• expose MHCflurry.fetch() and MHCflurry_Affinity.fetch() class helpers

Verification

• ./lint.sh
• ./test.sh (692 passed, 53 skipped, 2 xfailed)
• exercised installed console entry points: mhctools ls, mhctools ls --json, mhctools fetch calis, mhctools fetch mhcflurry