This reverts commit a4f5f8f0.

More aggressive combination of diagonal operators

See merge request ift/NIFTy!235

Fix draw sample

See merge request ift/NIFTy!236

# Conflicts:
#	nifty4/operators/endomorphic_operator.py

- 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)