Need to implement mcmc_fixed flag in @estimates

[Craig44]

Currently this is disabled due to the covariance including all parameters and then the MCMC sampling with less parameters than in the covariance.

To get around this users need to edit both input values and @estimate blocks to replicate this functionality which is prone to errors.

In order to implement this we need a switch on parameters saying is mcmc_fixed and ignore any adjustments to the parameter during mcmc, slightly inefficient as we will be sampling from a larger covariance matrix, but is a quick and dirty.

You will need to adjust the loaders to account for parameters estimated but fixed at mcmc i.e. the MPD report.