Add 'cov' argument to mxSE
Add 'cov' argument to mxSE

This lets users supply their own free parameter covariance matrix
instead of relying on the inverse Hessian returned from optimization.

In particular, this lets users supply the
 * bootstrapped parameter covariance
 * robust parameter covariance from imxRobustSE
 * empirical parameter covariance from a Monte carlo simulation

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