Add reflective boundary conditions

This MR generalises the periodic boundary conditions to allow reflective boundary conditions.

WIP because I only made it work with dynesty in the first case. Hopefully, this will help sample to the edges of priors for zenith angles.

test/sampler_test.py

@@ -156,7 +156,7 @@ class TestDynesty(unittest.TestCase):
                         logl_max=np.inf, add_live=True, print_progress=True, save_bounds=True,
                         walks=10, update_interval=300, print_func='func')
         self.sampler.kwargs['print_func'] = 'func'  # set this manually as this is not testable otherwise
-        self.assertListEqual([0], self.sampler.kwargs['periodic'])  # Check this separately
+        self.assertListEqual([0, 1], self.sampler.kwargs['periodic'])  # Check this separately
         self.sampler.kwargs['periodic'] = None  # The dict comparison can't handle lists
         for key in self.sampler.kwargs.keys():
             print(key)