Mass ratio distributions

implements mass ratio distributions and an attribute transformation to compute secondary mass from primary and mass ratio.

Fixes #16 (closed).

To Do

• implement BetaDistributedAsymmetricMassRatio
• write tests of new distributions
  • test-mass1-distributions
  • test-mass2-distributions
  • test-component-mass-distributions
  • test-mass-ratio-distributions
    • power-law q seems to be quite broken
    • beta-distributed q seems to work ok
• write tests of attribute transformations? The mass transformations are simple enough that I'm not going to write full unit tests for them
  • I wrote some tests by hand to make sure that the transformations and inverse transformations actually undo eachother: ~tests/test-mass-transforms
• write tests of parser (now splits strings by default to make lists, does this break anything else?) the parser logic is simple enough that I'm not going to write a full unit test for it. Plus, the de facto tests within the naming-conventions script still pass.