Fix LazyLinearOp.__add__/__sub__ and __getitem__

- For add/sub: using + and - operators instead of named functions in lambda expression to make possible calling universal functions (otherwise LazyLinearOperator-s based on a numpy array and a scipy matrix -- in this order -- fail to be added or subtracted.)
- For __getitem__ the result shape wasn't properly calculated.

Minor change in Faust.__getitem__: reformatting.