compute average RHS calls for reactions

[zingale]

It would be a useful diagnostic to compute and store / report the average number of RHS steps taken by the reaction network integrator each step.

[maximumcats]

Here is one way we could do it for Strang-split burning in react_state:

    if (verbose > 1) {

        Real avg_work_done = w.sum(0) / grids.numPts();
        amrex::Print() << "Average reaction work done per zone = " << avg_work_done << std::endl << std::endl;

    }

This takes advantage of the weights MultiFab we constructed for the reactions load balancing.

But for SDC this isn't quite as simple.

[zingale]

to be clear... this would work with the simplified SDC, right?

[maximumcats]

Yes, a similar thing should work for simplified SDC.