Cleaned out the OpenMP statements

Removed all the OpenMP pragmas, OpenMP functions and header inclusions. Also replaced the associated atomic operations with normal operations.

There might have been some race condition problems in some routines of space.c where some non-atomic operations were performed in critical parts of the code. Not sure whether it was actually a problem.

Closes issue #20.

See merge request !28

Former-commit-id: c8c82f2cf89aa6d3dd6a7ac76addf932ed1ccc0f