Add in ID setting for StructuralUnit

**Is your feature request related to a problem? Please describe.**
Allow the ID of StructuralUnits to be set, so that when Trajectories are constructed, atoms and molecules can have the same ID when they are included in future Configurations.

Start by locating the Python definition of StructuralUnit and the code that constructs Trajectories and Configurations. Trace how IDs are currently assigned or preserved, then verify that an explicitly set StructuralUnit ID allows atoms and molecules to share an ID in future Configurations.

Written by the indexing model from the issue text.