v1.1 - Performance and plotting enhancements

• Allow covariance to be manually specified in KDE constructor.
• New KDE.from_hist() method for constructing KDEs based on existing distributions (instead of finite points).
• Deprecated CDF functionality removed (for the time being).
• Triweight distribution works.
• Significant PDF evaluation speed improvements using numba
  • Sampling and evaluation code simplified.
• Use abc.ABC base classes
  • e.g. Distribution(object) ==> _Distribution(abc.ABC)
• Plotting improvements
  • BUG: fix incorrect label in rotate bottom-right panels of corner plots
  • Allow target lines to be drawn on corner plots using Corner.target()
  • Add arguments to limit the number of carpet and scatter points drawn