Check order of accuracy for time integration schemes at order > 2

The redesigned time-stepping operators "TimeOp" use unit tests to check validity.

There is also a check implemented which observes the convergence order by using different time step sizes. The issue with this check is that for 3rd and 4th order both, BDF and IMEX, schemes do not show convergence order > 2.

This is likely due to restarting from a lower order scheme which consequently influences the global error and likely causes the reduction. We could fix this by implementing a multi-stage scheme which allows restarting without using lower-order schemes.

We can address this issue once we have an implementation for a Runge-Kutta type scheme.