Gpu block size

If the Assignments act on 2D fields but 3 GPU indexing parameters are provided, an error occurs. With this MR the third parameter is fixed to 1

requested review from @hoenig

assigned to @holzer

added 3 commits

• 503be23d...343d0708 - 2 commits from branch pycodegen:master
• 1465fe2c - Merge branch 'master' of i10git.cs.fau.de:pycodegen/pystencils into GPUBlockSize

Compare with previous version

enabled an automatic merge when the pipeline for 1465fe2c succeeds

merged

mentioned in commit a100881c