Minor change to how non standard parameters are checked

Previously, parameters in fill_prior was only ever used to fill in the non-standard parameters if sampling in something other than the defaults. This directy checks that. The advantage is a cleaner logic and we no longer assume the likelihood has a non_standard_sampling_parameter_keys attribute.

test/prior_tests.py

@@ -198,6 +198,7 @@ class TestFillPrior(unittest.TestCase):
     def setUp(self):
         self.likelihood = Mock()
         self.likelihood.parameters = dict(a=0, b=0, c=0, d=0, asdf=0, ra=1)
+        self.likelihood.non_standard_sampling_parameter_keys = dict(t=8)
         self.priors = dict(a=1, b=1.1, c='string', d=tupak.prior.Uniform(0, 1))
         self.priors = tupak.prior.fill_priors(self.priors, self.likelihood)