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

@@ -263,6 +263,7 @@ class TestInterferometer(unittest.TestCase):
         self.outdir = 'outdir'
 
         self.injection_polarizations = dict()
+        np.random.seed(42)
         self.injection_polarizations['plus'] = np.random.random(4097)
         self.injection_polarizations['cross'] = np.random.random(4097)