scipy deprecation warnings in stats6d.py

[bruhwiler]

/rsbeams/rsbeams/rsstats/stats6d.py:91: DeprecationWarning: scipy.zeros is deprecated and will be removed in SciPy 2.0.0, use numpy.zeros instead

/rsbeams/rsbeams/rsstats/stats6d.py:18: DeprecationWarning: scipy.average is deprecated and will be removed in SciPy 2.0.0, use numpy.average instead

/home/vagrant/jupyter/repos/rsbeams/rsbeams/rsstats/stats6d.py:37: DeprecationWarning: scipy.sqrt is deprecated and will be removed in SciPy 2.0.0, use numpy.lib.scimath.sqrt instead