names backend

This patch implements dynamic renaming so that the library can play nicely with different naming conventions (e.g., "mass1_source" vs. "mass_1_source"). This involved pushing around quite a bit of stuff.

• Fixes #10 (closed).
• Fixes #11 (closed) (by removing chi_p altogether).
• Fixes #13 (closed).

This has been tested with an updated test suite. All tests can be run via ~/tests/test, which includes tests of the naming convention logic as well as sampling distributions.