Inserting checkpoint at current time

Bug report

What happened / Problem description:

Inserting a checkpoint into the time loop at the current simulation time (e.g. setting a checkpoint at t=0 when current time is 0) works, but sets the current time step size to 0.

This leads to NaNs in the storage evaluation and the simulation fails after exceeding the maximum amount of time step reductions.

What you expected to happen:

That no check points at the current time are inserted and a warning is shown that notifies the user of this. Currently this only happens when checkpoint time < current simulation time.

Additionally, a check that makes sure that dt > 0 while the simulation is not finished would also be useful in my opinion.

Environment:

• DuMux version: master