further optimizations for spatialstateupdater

[moritzaugustin]

parallelize the tridiagonal system's solution for the cable equation using existing cyclic reduction code or similar algorithms: https://code.google.com/archive/p/tridiagonalsolvers/source/default/source

[moritzaugustin]

alternative optimization would be to use an explicit time discretization avoiding the need of solving the linear systems in each timestep. however: we would need a very small timestep particularly for many compartments in a not very large branch (cf. also https://en.wikipedia.org/wiki/Courant%E2%80%93Friedrichs%E2%80%93Lewy_condition)

[moritzaugustin]

and potentially also the coupling system could be parallelized though not using a standard parallel tridiagonal solvers such as cyclic reduction but rather exploiting the specific graph structure in the hines (coupling) matrix