Added `duration`, `sampling_frequency` and `start` time properties to the `Interferometer` class

bilby/gw/detector/interferometer.py

@@ -38,6 +38,9 @@ class Interferometer(object):
     vertex = PropertyAccessor('geometry', 'vertex')
     detector_tensor = PropertyAccessor('geometry', 'detector_tensor')
 
+    duration = PropertyAccessor('strain_data', 'duration')
+    sampling_frequency = PropertyAccessor('strain_data', 'sampling_frequency')
+    start_time = PropertyAccessor('strain_data', 'start_time')
     frequency_array = PropertyAccessor('strain_data', 'frequency_array')
     time_array = PropertyAccessor('strain_data', 'time_array')
     minimum_frequency = PropertyAccessor('strain_data', 'minimum_frequency')