Entangled units

[ddahlbom]

Adds basic "entangled units" functionality to Sunny. Usable with existing optimization, classical dynamics and spin wave code. The functionality is sufficient for basic examples and is not yet public facing. The system creation function can only be accessed as Sunny.EntangledSystem.

While this is safe as a non-public feature, the following steps remain:

1. Fill out test suite
2. Test on systems with units larger than dimers
3. Stress test reshaping
4. Finish the program of incorporating position information in the MeasureSpec so the EntangledSpinWaveTheory can be reduced to an ordinary SpinWaveTheory. This will reduce code duplication.