Add more diagnostic methods and tests for bayesnecfit objects

[beckyfisher]

At the moment most diagnostic tests only check for model convergence. Give the sensitivity of NSEC on estimates of variance, diagnostic tests to ensure that the model simulated variance is representative of the input data would provide surety that the estimates are valid.

[beckyfisher]

IN the first instance we could implement a pp_check method for the bayesnecfit class and include some examples in the single model usage vignette.