lalinference has undeclared dependency on scikit-learn (sklearn)

The python-lalinference package has a dependency on scikit-learn that is currently undeclared:

$ git grep -E "^(from|import) sklearn" lalinference/python/
lalinference/python/rapidpe_calculate_overlap.py:from sklearn.neighbors import BallTree
lalinference/python/rapidpe_compute_intrinsic_grid.py:from sklearn.neighbors import BallTree

However, scikit-learn is not available on Scientific Linux 7, so we have two options

1. remove the relevant code from lalinference into a separate package that is not part of lalsuite, but depends upon lalinference
2. package up scikit-learn for rhel7

@chris-pankow, you are the last contributor to the two relevant scripts, can you comment on this, please?