Allow setting container selections directly in the MET maker

I noticed that the MET maker doesn't do its own selection of objects, requiring the sequencing of several extra algorithms and the creation of several superfluous shallow copies. This MR introduces those selections so all that logic can be done internally to the MET algorithm.

This change should be backwards compatible: if a user provides a preselected container and no selection decoration the tool will behave in exactly the same way as before.