Add ConditionalFieldAccess (Field.Access after out-of-bounds check)

Adds a wrapper around a Field.Access that the access is only performed if a certain condition is met.

If I use this, I can safely perform calculations and adjoint calculations with ghost_layers=0 and obtain the correct gradients without separate boundary handling.

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added 9 commits

• 8dfe844c...ae69d3d3 - 8 commits from branch pycodegen:master
• 800eaf6c - Merge branch 'master' into 'ConditionalFieldAccess'

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cherry-picked into master

closed