updated to PR 1700
eliminated is_TensAdd and is_TensMul
eliminated forgotten is_TensAdd; eliminated isinstance calls in TensExpr
Kept `__hash__` only for `TensAdd` and `TensMul`, used `super`.
Improved the documentation; made attributes private; _pretty for TensorHead;
renamed in TensMul rank ext_rank and added rank.
use private attributes in code
eliminated explicit call to subclasses in TensExpr;
fixed bugs in _TensorManager
Updated to PR 1700
Code clean up, fixed some bugs and improved coverage.
cleaned up the tests
Updated to PR 1700
updated to PR 1700