Design interface for choosing selecting square root branches in K-matrix

[redeboer]

The current (experimental) implementation of K-matrix dynamics doesn't consider which square root to take (i.e., there is no control over which Riemann sheet is chosen).

An idea might be to add a 'switch Symbol' to ComplexSqrt which can be substituted to ±1. For each channel, there is one such switch Symbol, leading to 2^n combinations (Riemann sheets).

Part of epic #67