- moved draw_sample() and inverse_draw_sample() to EndomorphicOperator
- inverse_draw_sample() is always possible if draw_samples() is implemented (see EndomorphicOperator)
- changed WienerCurvature draw_sample() to probably better version
- implemented both functions in DiagonalOperator and ScalingOperator properly (i.e. check for positive definiteness)

Fixed a few small bugs.

Reordered methods in space classes.
Made Space.weight to an abc.abstractmethod.

Moved unitary property to LinearOperator.

Restructured new operator classes -> Introduced endomorphic_operator.py in order to avoid violation of Liskov's SP for square- and diagonal operator.
Removed paradict from operators.
Added from_random as static method to Field.