Support dynamic time-step sizes

[davidscn]

The implemented solvers only support constant time-step sizes. We added asserts to prevent misuse. The time-stepping schemes could theoretically handle dynamic time-step sizes, but within the solvers, we create initially dependent data structures once, such that a dynamic time-step size invalidates these data structures. Examples:

https://github.com/precice/dealii-adapter/blob/dbb25bea51531b7e4e0c9b5e4def3a7fadf8367c/source/nonlinear_elasticity/include/nonlinear_elasticity.h#L241-L250

https://github.com/precice/dealii-adapter/blob/dbb25bea51531b7e4e0c9b5e4def3a7fadf8367c/source/linear_elasticity/linear_elasticity.cc#L350-L351