Modify inject signal

This restructures some functionality surrounding the inject_signal method.

• Added inject_signal_from_waveform_generator and inject_signal_from_waveform_polarizations that are slightly less confusing to use than inject_signal.
• Removed if/else statement that checks size of the injected signal. The injection will now just fail if that condition is now not fulfilled. It was already breaking before, but at a different place.
• Added explicit tests for these methods.

test/detector_test.py

 from __future__ import absolute_import
 
 import bilby
+import inspect
 import unittest
 import mock
 from mock import MagicMock
@@ -529,7 +530,8 @@ class TestReferenceInterferometerList(unittest.TestCase):
     def test_test(self):
         if sys.version_info < (3, 0):
             return
-        reference_ifos = bilby.gw.detector.InterferometerList.from_hdf5('bilby/reference_ifos/ifo_H1L1V1.h5')
+        current_dir = os.path.dirname(os.path.abspath(inspect.getfile(inspect.currentframe())))
+        reference_ifos = bilby.gw.detector.InterferometerList.from_hdf5(current_dir + '/reference_ifos/ifo_H1L1V1.h5')
         self.assertEqual(reference_ifos, self.ifos)