Fix problem with Prior prob and ln_prob if passing multiple samples

The changes in !400 (merged) (in c7b36e76) broke the Grid likelihood as the prob and ln_prob prior methods would not work if multiple samples are passed to them. This MR fixes the problem.

cc @colm.talbot

bilby/core/prior.py

@@ -277,10 +277,10 @@ class PriorDict(OrderedDict):
                 else:
                     return 0.
             else:
-                cprob = np.zeros_like(prob)
+                constrained_prob = np.zeros_like(prob)
                 keep = np.array(self.evaluate_constraints(sample), dtype=bool)
-                cprob[keep] = prob[keep]
-                return cprob
+                constrained_prob[keep] = prob[keep]
+                return constrained_prob
 
     def ln_prob(self, sample, axis=None):
         """
@@ -310,10 +310,10 @@ class PriorDict(OrderedDict):
                 else:
                     return -np.inf
             else:
-                cln_prob = -np.inf * np.ones_like(ln_prob)
+                constrained_ln_prob = -np.inf * np.ones_like(ln_prob)
                 keep = np.array(self.evaluate_constraints(sample), dtype=bool)
-                cln_prob[keep] = ln_prob[keep]
-                return cln_prob
+                constrained_ln_prob[keep] = ln_prob[keep]
+                return constrained_ln_prob
 
     def rescale(self, keys, theta):
         """Rescale samples from unit cube to prior