Epic: Major refactoring

[aimalz]

In the interest of time, I'm splitting #79. What I can't do in time for an immediate code release to coincide with the paper, I will address in a future version.

The time for reach goals is now! I'm migrating content from #79 now that the paper is all done. Here's what I want to do next:

• 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
• replace qp.PDF.\[insert parametrization format here\]with cleaner dict object
• remove checking if parametrization exists/calculating with default parameters in favor of exceptions
• Rethink qp.PDF.limits #95
• find a way around infty for converting quantiles to other parametrizations
• Better quantile spacing #35
• rename histogram parametrization to stepfunc to reduce confusion
• make the piecewise constant distribution consistent in interpolation, integration, etc.
• formalize interpolation options
• Better handling of samples #88
• check for consistent style in documentation
• clean up excessive print statements