GP scalar use case (new design)

Thought this approach was a little more lightweight than the one in #208.  Things left to do

1. Likelihood evaluation;
2. Refactoring of the hyperprior parameters so they're input-file-tweakable;
3. Possibly adding a new constructor to the `StatisticalInverseProblem` object to take a `GaussianProcessHelper` instantiation and 'do the right thing'.

This is a work in progress.