DiscreteDistribution keeps link to parent ContinuousDistribution

[sbenthall]

fixes #949

• New ContinuousDistribution class
• approx method on this class calls a method pmf_X (should be renamed...) which is overridden by subclasses
• DiscreteDistributions created with approx link back to their parent continuous distribution with arguments to approx method
• new tests for this functionality, changelog update included.

Please ensure your pull request adheres to the following guidelines:

• [x] Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• [x] Updated documentation of features that add new functionality.
• [x] Update CHANGELOG.md with major/minor changes.