eliminated is_TensMul and is_TensAdd; added default commutator group
to _TensorManager for noncommuting tensors.
eliminated isinstance calls in TensExpr
Improved the documentation; made attributes private; _pretty for TensorHead;
renamed in TensMul rank ext_rank and added rank.
eliminated two __hash__
use more private attributes
Eliminated explicit call to subclasses in TensExpr;
fixed bugs in _TensorManager