Refactor strain data

Part 3 in the detector overhaul. Builds on !473 (merged)

This generalizes the use of the PropertyAccessor to InterferometerStrainData and WaveformGenerator

Keeps everything nice and backwards compatible

test/gw_example_test.py

@@ -43,9 +43,9 @@ class Test(unittest.TestCase):
 
     def test_examples(self):
         """ Loop over examples to check they run """
-        examples = [#'examples/injection_examples/fast_tutorial.py',
-                    #'examples/injection_examples/marginalized_likelihood.py',
-                    '../examples/open_data_examples/GW150914.py'
+        examples = ['examples/injection_examples/fast_tutorial.py',
+                    'examples/injection_examples/marginalized_likelihood.py',
+                    'examples/open_data_examples/GW150914.py'
                     ]
         for filename in examples:
             print("Testing {}".format(filename))