Adds a object backend for optimized contractions on arbitrary Python objects.

New Features

• (#145) Adds a object based backend so that contract(backend='object') can be used on arbitrary objects such as SymPy symbols.

Enhancements

• (#140) Better error messages when the requested contract backend cannot be found.
• (#141) Adds a check with RandomOptimizers to ensure the objects are not accidentally reused for different contractions.
• (#149) Limits the remaining category for the contract_path output to only show up to 20 tensors to prevent issues with the quadratically scaling memory requirements and the number of print lines for large contractions.