Structure_Engine: Composite section implementation

There are currently no constructors for composite sections. This also means that the methods using composite sections can not be tested.

Along with this, a general review of the composite section might be required.

The issue names no files or tests. Start by locating the composite section definitions and the methods that use them, then inspect how related sections are constructed and tested. Done means composite sections have usable constructors, their dependent methods can be tested, and the requested general review has been completed.

Written by the indexing model from the issue text.