Covmat -- v3

[stevencarlislewalker]

• (breaking change) global rename neg_bin -> nbinom? #342
• explicit singularity check for parameter covariance matrix before simulating from the distribution of parameter uncertainty -- use the check in MASS::rmvnorm
• New function mp_opt_attempted indicating if an attempt has been made to calibrate model parameters through optimization of a likelihood function or posterior density
• New function mp_uncertainty_estimated indicating if a model contain estimates of parameter uncertainty, probably through a covariance matrix
• Breaking change : mp_trajectory_replicate now uses parameter uncertainty to generate randomness, in addition to 'process error'