Harmonise devito's adjoint implementation with standards in AD literature

[navjotk]

Devito follows slightly twisted indexing schemes, that are not the same as the standard ones seen in AD literature. We added an extra forward step in pyrevolve to get around that. However, now that we're trying to draw generic conclusions from this implementation, we need to remove this difference to decouple pyrevolve from devito's idiosyncracies. Ideas on how to remove this hard-coded bit?