Add Truncated Normal Distribution

[BenHewins]

This is still a work in progress, I'd welcome comments, corrections, guidance, etc.

This pull request implements a truncated normal distribution as described on Wikipedia and the references in that article.

Still to do:

• Some of the methods and properties lack tests/do not quite pass existing tests
• The sampling implementation used at the moment is very naive and better ones exist elsewhere
• Skewness is not implemented yet (I'm not sure what the correct formula is yet)
• Cannot handle standard deviations of exactly 0, I think this will require special cases for most of the descriptive statistics if it is allowed at all.
• Cumulative distribution function has issues with floating point errors in current implementation.