Update grid_subset_index in ggd quantities with mesh extension

[anchal-physics]

Please see ProjectTorreyPines/FusionSyntheticDiagnostics.jl@086e9a7

While extending mesh extension, when the grid_subset with index 5 is changed into index -5, all ggd quantities that refer to the grid_subset_index 5 should also be updated to preserve the correct pointing. This should be an easy fix and should go in any existing WIP PR.