SE Calculations for `as.lmitt` vs. `lmitt`

1) `DirectAdjusted` objects should have a slot indicating whether it was generated using `lmitt` or `as.lmitt`.
2) When calling `vcovDA` on a `DirectAdjusted` object generated using `as.lmitt`, return the full covariance matrix if model-based SE's are called for, otherwise error.

Start by finding the DirectAdjusted and vcovDA implementations, then trace how lmitt and as.lmitt create DirectAdjusted objects. Done means objects retain which construction path was used and vcovDA returns the full covariance matrix for model-based SEs from as.lmitt, or errors otherwise.

Written by the indexing model from the issue text.