Faster Model Building

• Improved model building, by removing check for colliding variable names.
• Better API for getting variable values via solver.get_values(vars) which returns the same dicts as before. This works without caring of variable names.
• Old get_variable_values(...) marked as deprecated. Switch to get_values(...).
• Improved upper-bounds for edges variables in s-t walks.
• Fixed issue where is_solved() returned None for models that times out.
• Fixed type declarations, that made it incompatible with Python 3.8

First support for cyclic graphs

• Support for cyclic graphs and decompositions into S-T walks
• Support for subset constraints for these walks
• Support for safety optimizations for S-T walks, computed via dominators
• Support for MinFlowDecomp, k-LeastAbsErrors, k-MinPathCover
• Updated documentation for these cyclic models