Adding unit tests for vectors

Todo :

• Test all abstract methods
• Correct typo in TorchVector.maximum()
• Test most other methods (basic operators, pack/unpack, __repr__)
• Test register_vector_type
• Correct NumpyVector.sum() ans test repercussions

Dropped:

• When running on a Mac M1, as of jan 2022, abort error in test/model/test_tflow.py. this is due to a C compile error, will report the issue on github.

Remarks :

• The Vector.build() static method is not used anywhere ?