qnm: A Python package for calculating Kerr quasinormal modes, separation constants, and spherical-spheroidal mixing coefficients

Archived version (v0.4.0) to match JOSS publication.

Changes since v0.3.0:

• Rootfinding tolerance and continued fraction tolerances are now independent
• Cache directory location determined by a more standard procedure and can be controlled by env var, see docs of qnm.cached.get_cachedir()
• Using pathlib (or pathlib2) for cross-platform path access
• Add options interp_only and resolve_if_found to KerrSpinSeq.__call__ for better control of (re)solving
• Start versioning cache data
• Start using Travis for automated tests
• Add notebook for comparison against Greg Cook's precomputed tables