Give FlashFluxRegisters ways to accumulate data in registers (AMReX-C…

…odes#3597)

I introduce variant methods that allow adding to previously stored data
in Flash(-X) flux registers, rather than just copying over the older
data.

For Flash-X'ers: Something like this is needed to get flux correction
working in the nontelescoping variant of the current Spark Hydro
implementation.

## Summary

## Additional background

## Checklist

The proposed changes:
- [ ] fix a bug or incorrect behavior in AMReX
- [x] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

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Co-authored-by: Weiqun Zhang <weiqunzhang@lbl.gov>