[Enhancement] User selected solver for PFR reactor (DotNumerics)

[andr1976]

Currently the implicit Runge-Kutta solver from DotNumerics is applied. However, the system is not necessarily stiff e.g. for single reaction systems. In that case the explicit Runge-Kutta is faster. E.g. the two reactors in #629 solves twice as fast with the explicit method