After filling a new fine grid solution via linear interpolation, kappa*q may need to be conserved rather than q in the presence of a capacity function. This routine calculates the discrepancy in kappa*q and modifies q to account for it.
The inputs are solution and aux data (val, aux) from a fine grid that has just been filled via linear interpolation, as well as from a coarse patch (valc, auxc) that covers the fine grid.