Fix access to global_debye_length with tiling

[dpgrote]

The calculation of the global Debye length during binary collisions was added in PR #5763. It was recently realized that the access was being done incorrectly, not taking into account that the MFIter loop was using tiling. With tiling, the i_cell index is relative to the tile and cannot be directly used to access MultiFab data. The fix is to use the global_index that was add in PR #6954.

Note that this error was not detected since the CI test does not use tiling.

[EZoni]

Note that this error was not detected since the CI test uses a domain uniformly filled with plasma so accessing the global Debye length data at the wrong cell index did not have a significant effect on the results since it will also be uniform.

By chance, do you know if there is an easy way to modify the CI test such that the plasma doesn't fill the domain uniformly (so as to catch bugs like the one fixed in this PR going forward)?

[dpgrote]

Note that this error was not detected since the CI test uses a domain uniformly filled with plasma so accessing the global Debye length data at the wrong cell index did not have a significant effect on the results since it will also be uniform.

By chance, do you know if there is an easy way to modify the CI test such that the plasma doesn't fill the domain uniformly (so as to catch bugs like the one fixed in this PR going forward)?

@EZoni The test could be added, but would require somewhat significant change to the existing test.