Resolve #430 (Add normalisation flag to constrained prior)

This MR resolves issue #430 (closed) to properly normalize ConstrainedPriorDict.prob() when sampling the prior. This MR allows for users to input a normalisation flag when creating an instance of a Constraint prior and when calling ConstrainedPriorDict.prob() if there are normalisation constants given will return the normalisation*prob and otherwise use the ratio of total samples to kept samples as this normalisation constant. Pass normalisation=1 when creating a Constraint Prior for behaviour before this MR.

bilby/core/prior/base.py

@@ -438,7 +438,8 @@ class Prior(object):
 
 class Constraint(Prior):
 
-    def __init__(self, minimum, maximum, name=None, latex_label=None, unit=None):
+    def __init__(self, minimum, maximum, name=None, latex_label=None,
+                 unit=None):
         super(Constraint, self).__init__(minimum=minimum, maximum=maximum, name=name,
                                          latex_label=latex_label, unit=unit)
         self._is_fixed = True