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

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