Pdf improvements - Githubissues

Converted DensityEstimator to an abstract base class.
Added a cdf() method to density estimators for calculating the cumulative distribution function.
Redesigned the interval() method of DensityEstimator and changed the name of the frac argument to fraction to match naming with sample_hdi.
Renamed the GaussianKDE instance attribute s to sample to standardize naming across density estimator classes.
Various improvements to type-hinting in the pdf module.

C-bowman / inference-tools