Speed up core.prior classes

To solve issue #416 (closed)

bilby/core/prior.py

@@ -1814,8 +1814,7 @@ class StudentT(Prior):
         -------
         Union[float, array_like]: Prior probability of val
         """
-        return gamma(0.5 * (self.df + 1)) / gamma(0.5 * self.df) / np.sqrt(np.pi * self.df)\
-            / self.scale * (1 + ((val - self.mu) / self.scale) ** 2 / self.df) ** (-0.5 * (self.df + 1))
+        return np.exp(self.ln_prob(val))
 
     def ln_prob(self, val):
         """Returns the log prior probability of val.