Finalized LinearOperator base class. Added a first version of a square operator.

nifty/nifty_utilities.py

nifty/operators/__init__.py

nifty/operators/linear_operator/__init__.py

nifty/operators/linear_operator/linear_operator.py

nifty/operators/linear_operator/linear_operator_paradict.py

nifty/operators/operator/operator.py

nifty/operators/square_operator/__init__.py

nifty/operators/square_operator/square_operator.py

nifty/operators/square_operator/square_operator_paradict.py