Created a TIDS object (Tensor Internal Data Structure), in order to separate the user representation of a tensor, from its internal representation.
Added missing docstrings to TIDS.
Added docstrings to tensor.rst file
Added more examples and expanded docstring of TIDS
Merge pull request #2442 from Upabjojr/TIDS

TIDS (Tensor Internal Data Structure)
Patches to fix the non-compliance of the tensor module with test_args tests.
removed commented out code
Merge pull request #2445 from Upabjojr/fix_tensor_args

Fix some defects in tensor module.
Added code for coordinate variables handling
Added tests, changed express function
Rectified typos
Added cache for dcm
Rectified errors, added docs for CoordinateSym
Rectified ReferenceFrame doctest
Changed dcm()
Removed recursion in dcm()
PEP8 changes, added equality conditions for CoordinateSym and tests
Added tests for variable_map, resolved errors
Changed failing test
Removed TIME