Draft: [FIX] Index fields exclusively containing coordinates are dropped by code generator

Index fields that exclusively contain coordinate data (members x, y and z) and that are not explicitly accessed in the kernel assignments are dropped by pystencils.cpu.create_indexed_kernel in cpu/kernelcreation.py, prev. line 119. Then in line 128 the list of index fields is empty, and the code generator finds no field containing the coordinate information. Code generation then aborts.

Is there a reason why index fields are first filtered this way?