Remove atomics and openMP

[taupalosaurus]

I removed as much as I could of code needed for threading:

• all atomic objects/operations
• classes that use these and are not needed anymore
• openMP pragmas and headers
• some unused code that seem to be remnants of the threaded past

I did not change the algorithms themselves, even if some ways of looping could be improved now. That requires more work. So the behavior of the code should be absolutely unchanged.

[taupalosaurus]

merged. Thanks @ggorman.