Refactor database

WARNING: migration will wipe most djsopnet_ tables

Remove user/create/edition times from purely sopnet tables
Change slice and segment hashes from strings to 64-bit integers
Slice and segment hashes are now PKs and FKs
Add multi-column unique constraints and indices to many tables
Change feature values to double precision
Rename FeatureNameInfo model/table to FeatureInfo
Add feature weights
Conflict sets are now related to exactly two slices
Remove ConflictSetRelation model/table
Remove SegmentCost model/table (catsop does not store costs)
Require segments to have at least one slice
Remove redundant db_index directives from foreign key columns
Add user constraint models and view (WIP)
Generate a super node graph from a skeleton graph by collapsing consecutive nodes in the same section
Retrieve segments that are consistent with the slice sets associated with each node in the supergraph.
Updated Constraint models in Djsopnet. Constraint generation function added.