Heavily refactors sampleMass

We now use the new Chain, have no globals, and remove a bunch of needless repetition of variables
In addition, we provide a covariance-based sampler, and correctly convert sigma to variance.
Moves the more important changes from sampleMass to sampleWDMass

Primarily, this is the sigma to variance correction. Secondarily (though equally or more important for runs with Y or carbonicity) is the new reader for result data.
Fixes some size_t warnings
Fixes some bugs in sampleMass