Epic: Polishing for response to referee report

[aimalz]

I'm collecting things I really need to fix before a proper code release.

• make it easy for users to add formats by making a parametrization class (like scipy.stats.rv_continuous, etc.), with consistent syntax for conversions to replace the using keyword, opens the door for implementing SparsePZ, FlexZBoost formats punting this to a later code release
• eliminate qp.PDF.truth in favor of using mix_mod and qp.PDF.first
• rename histogram parametrization to something like piecewise constant or stepfunc to reduce confusion and make the piecewise constant distribution consistent in interpolation, integration, etc.
• remove checking if parametrization exists/calculating with default parameters in favor of exceptions
• find a way around infty for converting quantiles to other parametrizations
• formalize interpolation options and include option for user-defined callable punting this to a later code release
• check for consistent style in documentation
• clean up excessive print statements
• Better handling of samples #88 punting this to a later code release
• Expand format conversion options #65
• move qp.Ensemble.stack() and other "derived" functions to utils.py instead of within the class and publish demos for the "derived" use cases
• replace qp.PDF.\[insert parametrization format here\]with cleaner dict object punting this to a later code release

[aimalz]

This has been partially addressed by #93. Stay tuned for more soon!

[aimalz]

I'm closing this issue and moving over to #94 for future development with broader goals.