Switch index type from int32 to int64

For large domain sizes, int32 is not sufficient. Thus it is planned for waLBerla to change cell_index_t from int to int64. To make it consistent with pystencils and to prevent conversion warnings the index type for pystencils is also adapted to int64

Fixes lbmpy#18 (closed)

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• 94fbc877 - More adaptions

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• 63be89c8 - revoke changes on flag field

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Is this really necessary? You'd need 4 billion cells per Cartesian direction to make a cell index overflow. The smallest usable 2D system where this would happen would have 2^32 x 4 cells, which at single precision would occupy 64 GB of memory. That doesn't fit onto a GPU, while in CPU land you would already parallelize. As far as I can tell, the only indices that might be worth making 64 bits are linearized indices (as in zL^2+yL+x), see lbmpy#18 (closed).

This MR is closed because it was decided to not switch the data type of the flag field.

closed