Add OpenMP

I think it should be possible to rewrite the loops to include OpenMP pragmas. The final update could be protected with `#pragma omp atomic` and you can use `default(shared) private(tx ty ix iy) firstprivate(dataptr)` or something similar. You'll need to redo the loops, however, you'll need for loops instead of the inner while (or maybe the outer do while), and the stride part might need to be precalculated in some way.

No implementation file or test is named. Start by locating the C histogram loops and understand their current control flow and stride handling; define completion with a correct OpenMP-enabled implementation whose results match the existing histogram behavior and whose performance can be measured.

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