"Discrepancy off" mode

[roystgnr]

GPMSA needs to be able to disable discrepancy modeling entirely - discrepancy precision and discrepancy correlation strength will not be calibrated, and discrepancy terms will be zero (effectively discrepancy precision will be infinity)

[dmcdougall]

@briadam and I remember seeing some older pieces of code that check for 'disabled parameters' on the MCMC side. Let's see if we can revive some of that infrastructure to allow users the flexibility to turn off discrepancy rather than burying it in the GPMSA code.

[roystgnr]

Note to self - if we could find and fix up that code, it would also resolve #510.