Clean semidefinite.py

[Ericgig]

Description
Did some cleaning in semidefinite.py:

• Renamed functions that should be private: kron -> _kron, conj -> _conj, etc.
• Removed dag, and bmat: never used.
• Moved qudit_swap as swap in operator.py and added tests.
• Use functools.lru_cache instead of re implementing a memoize decorator. This used to be done in dnorm, comments were still present, removed.
• Merged herm pos_noherm, pos and dens into _make_constraints.
• Removed the logger call.
• black the file.

@hodgestar Now only qutip-ctrl use the logger, can we move logging_utils.py there?

[coveralls]

Coverage: 75.019% (-0.3%) from 75.328% when pulling ea33630 on Ericgig:misc.semi into ef81573 on qutip:master.

[hodgestar]

Looks good!

I suggested removing the SciPy intermediate step, a little because it's fast, but more because it removes the dependency.