[feature/refactor] add tileid function
Instead of asking the user to provide the offsets into a tiling, add a function providing a tileid. This tileid corresponds to the in-memory order of tiles, making the tilestart functions a lot simpler. We still need to split the tileid for tilestart because scratchpads create requests based on tileids. Also add a unit test for tiling_2d, to make sure we're not doing anything crazy.
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