Thread safety of anglib and problematic derived types for OMP

[vanderhe]

What is your suggested feature? Please describe.
To achieve at least thread parallelization of sktwocnt, two problems should be addressed:

1. anglib is not thread-safe and its module structure is at least questionable
2. OMP will be unable to copy the derived integrator type in the loop over dimer distances
   (workaround for testing: initialize integrator within the loop --> results in overhead due to recalculated finite differences matrices)