Implement spin density matrix

[redeboer]

This issue is twofold.

1. Convenience/performance. A wave represents one specific spin projection solution from the initial to the final state. Waves can be added coherently or incoherently. When all waves of the process/sub-process are added coherently, it can be more convenient to reduce this coherent addition operation into a matrix operation. See Implement Spin Density Matrix ComPWA-legacy#124 for a more details explanation. Note that this is purely a mathematical reformulation, but allows for a more compact amplitude model definition. It could also be beneficial for the tensor formalism to support such matrix operations and could also increase performance.

2. Currently ComPWA / Tensorwaves does not support the spin density matrix coherent addition of initial states. This would be a small physics feature than could be made available once part 1 is available.

The paper that discusses an analysis where such a mathematical reformulation in terms of matrices is beneficial is
https://arxiv.org/pdf/1809.04038.pdf