expanded functionality to support injection generation

This patch implements the following

• support for both time-domain and frequency-domain waveform generation. Caching is updated within Event objects accordingly
• estimates of template duration (require the waveform to have already been generated)
  • freq-domain: simple PN scaling between frequency (flow, fhigh) and time
  • time-domain: the length of the vector returned by lalsimulation.SimInspiralTD
• automatic selection of approximants based on masses
  • if approximant is already set within Event instance and no approximant (None) is passed to the waveform call, code automatically picks the waveform based on the component masses
    • can be done within transforms.detection.SNR based on source-frame masses (if available) or detector-frame masses
  • Note: not all waveforms are compatible with precessing spins, so we chose defaults that are.
• declares new sets of SamplingDistributions to generate macroscopic properties of neutron stars based on their EOS and their masses, eg p(Lambda|mass)
• support general mixture models
  • implement automatic mixture-model generation when subpopulations are instances of SamplingDistributions
  • implement automatic mixture-model generation when subpopulations are instances of EventGenerator (joint distributions)
  • expand config-parsing to automatically instantiate mixture models

To Do

• test matter distributions' behavior when we have multiple stable branches
• run unit tests for matter distributions
• test Mixture behavior
• test config parser when instantiating Mixture (only checked that it doesn't raise an error...)