Revisit mechanism supporting math operations involving `LinearOperator` objects

[bwohlberg]

The mechanism supporting math operations involving LinearOperator objects (specifically scalar multiplication and addition and subtraction of two LinearOperator objects) should be revisited to determine whether it can be simplified. Specific issues include:

1. The _wrap_add_sub wrapper defined in _linop.py duplicates the __add__ and __sub__ definitions in class LinearOperator.
2. It's not clear whether applying the wrapper to methods of classes derived from LinearOperator is really necessary.