Djsopnet: refactor assemblies to be corewise

Make assemblies, i.e., connected components of segments, a concept
specific to a core, with AssemblyEquivalences to indicate assemblies
adjoining at core interfaces in the future. Assemblies only reference
segments in solution, as assemblies are only coherent for a particular
solution. Add a view method to generate assemblies from the precedent
solution for a core.

Add relationships between assembly equivalences and their mapped
skeletons, and between treenodes of mapped skeletons and their
corresponding assembly slices. Remove old relationships between
assemblies, segments, and slices.