Allow for MCMC sampling without the detuned spring

This addresses @jeffrey-kissel's comment on #167 (closed). Evan and I had the idea that since the detuned spring model may be a poor choice for some reference model, we instead let the user specify if they want to run the MCMC without the detuned spring. This will then break the circular process of running MCMC on the reference measurement, then updating the parameters in the ini file for the reference measurement for later use in the stacking method.

I'll also revert the changes made to stack measurement to not allow the use of the reference parameters specified in the ini file (!180 (merged)).