Feature/spliced probability dist

[apmccartney]

@jlconlin Let's try our first github pull review.

This pull request is composed of

• A new class called SplicedProbabilityDistribution which provides continuous interpolation between two probability scheme, given their conditions and a queried condition. The intent of this class is to provide the machinery for the givenCondition method added to the BivariateProbabilityDistribution class (see below). The spliced probability distribution only stores references to the boundary probability distributions and hence its validity is limited to the scope of those objects.
• The BivariateProbabilityDistribution has been extended to provide a probability distribution object for an arbitrary queried condition value. This method has been labeled givenCondition and returns a unique_ptr<math::API::ProbabilityDistribution> which point to a spliced probability distribution.
• I cleaned up the BivariateProbabilityDistribution tests (the same way that was done to the EnergySpectrum in nuclear data).
• A README.md has been added with all the relevant badges.
• Minor revisions to logging messages to provide additional information or clarify cause of error.
• Minor revisions to documentation, cleaning up a handful of typos and providing clarification in some places.

[jlconlin]

Very nice. Just two comments about the documentation that should be addressed. They are very minor.

[apmccartney]

The documentation was cleaned up in the coming pull request.