Added some imports

tupak/gw/prior.py

-import tupak.core.prior
+from __future__ import division
+import tupak
 import os
+import logging
+import numpy as np
+from scipy.interpolate import interp1d
+from scipy.integrate import cumtrapz
+from scipy.special import erf, erfinv
+import scipy.stats
 
 
 class UniformComovingVolume(tupak.core.prior.FromFile):