Scalar GPMSA use case

This pull request is a work in progress for implementing the scalar use-case for GPMSA.  See #173.

The current state:

1. An example scalar gp statistical application is written and builds with the build system
2. A new constructor was added to the `ExperimentModel` class that doesn't require the user to pass K and D matrices.  This is currently a stub to make the example compile.  The next step is to implement this.  Future work would be to use this constructor for any GP use-case.  Asking the user to provide those matrices by hand is superfluous; they can be constructed by `QUESO` under the bonnet from the data.
3. The experiment/simulation data is hard coded in the example.  For now this is fine since there's O(30) data points.  It'd be nice to read these in from a file, though.

Next big step is the actual scalar implementation.  I just did the statistical application to get an API I was somewhat happy with.  __This PR should not be merged until the full scalar implementation is complete.__