Fix/clarify covariance matrix in NonLinearLSQFitter 

This PR does a variety of fixes dealing with how `NonLinearLSQFitter` returns information about the covariance matrix of the parameters.

Previously, this was being placed in the `fit_info` dictionary as something called ``'status'``.  I think that was just a typo/misunderstanding of what this return value means. (perhaps @nden can confirm?)  In fact, it's actually a value that's a scale factor off from the covariance matrix of the parameters.

However, it's non-trivial how you get from this return value to the *actual* covariance matrix of the parameters.  This covariance matrix is actually very important for estimating the errors on the parameters, so I think it's important this be made easy to get for users.  Hence, this also adds code to compute the properly-scaled covariance matrix estimate, and adds documentation explaining that the `fit_info` attribute has this information.