Added clear_current_row() to TableHandler in case the current time step is rejected.

With variable time stepping, the current time step is sometimes rejected. If TableHandler is used to track information about time steps, one needs to clear the information gathered so far and begin again. This function does that.

I'm open to changing the name of the function if another name is more suitable.
Can you write this as ``` n = std::max(n, p->second.entries.size()); ``` instead?