Parameter Update for Single Component Multivariate Normal with Missingness.
fit.mvn.miss.update(split_data, theta, fix_mean)
Data partitioned by missingness.
List containing the current `mean` and `cov`.
Fix the mean to its starting value? Must initialize.
List containing:
The updated `mean` and `cov`.
The initial `old_obj` and final `new_obj` EM objective.
The increase in the EM objective `delta`.