Adding simplified gpmsa api

This API feels more light-weight.  There's fewer objects that need
instantiating.  Everything is essentially in one GaussianProcessHelper object.
All the simulation and experimental data is in there, and I also envisage
having the GaussianProcessHelper object be a subclass of a ModelEmulatorHelper
object of some kind so in future releases we can support PCE.  Class names are
up for discussion.  Perhaps 'helper' is a bad name.  Maybe GaussianProcessData
instead?
Adding a prefix to the GP helper constructor
Adding a GaussianProcessHelper class
Document adding multiple simulations/experiments
Implement adding multiple simulations/experiments
Add initial pass at prior machinery
Add method to return gpmsa joint prior

This prior includes all the distributions for the hyperparameters.
Make GaussianProcessHelper a likelihood function

Gaussian processes are likelihoods.  This might be a better way of doing it.
Make error covariance matrix full size

This is the start of a move to allowing the user to pass in observation error
covariance matrices other than block diagonal
Remove ability to add experiments piecemeal

Bulk addition of *all* the experiments allows for the most general
structure of the observation error covariance matrix
Change name of gaussian process class

GaussianProcessHelper -> GaussianProcessEmulator.  Also rename gaussian
process source files to match this change
Make helper->emulator change for example
Fix full error matrix support and const issues
Fix includes and some templating errors
Add gaussian process emulator source to build sys
Add statistical inverse problem support for GPs

Also fix example to pass the posterior rv to the SIP (this should not have to
be done by the user, and is worthy of a separate fix)
Use a factory pattern
Fix all compile and link errors
Add logic for construcitng the GP after adds
Fix a whole bunch of segfaults