Determine reference_chirp_mass for MBGravitationalWaveTransient from prior when it is not specified

reference_chirp_mass of MBGravitationalWaveTransient should be less than the prior minimum of chirp mass to construct conservative frequency bands. This MR makes a small change on MBGravitationalWaveTransient so that it automatically uses the prior minimum of chirp mass if reference_chirp_mass is not specified. This also reorganizes tests for MBGravitationalWaveTransient and adds tests for this change.

test/gw/likelihood_test.py

@@ -1719,6 +1719,44 @@ class TestMBLikelihood(unittest.TestCase):
             abs(likelihood.log_likelihood_ratio() - likelihood_mb.log_likelihood_ratio()) / 2
         )
 
+    def test_reference_chirp_mass_from_prior(self):
+        """
+        Check if reference chirp mass is automatically determined from prior if no number has been passed
+        """
+        wfg_mb = bilby.gw.WaveformGenerator(
+            duration=self.duration, sampling_frequency=self.sampling_frequency,
+            frequency_domain_source_model=bilby.gw.source.binary_black_hole_frequency_sequence,
+            waveform_arguments=dict(
+                reference_frequency=self.fmin, approximant="IMRPhenomD"
+            )
+        )
+        likelihood1 = bilby.gw.likelihood.MBGravitationalWaveTransient(
+            interferometers=self.ifos, waveform_generator=wfg_mb,
+            reference_chirp_mass=self.priors["chirp_mass"].minimum,
+            priors=self.priors.copy()
+        )
+        likelihood2 = bilby.gw.likelihood.MBGravitationalWaveTransient(
+            interferometers=self.ifos, waveform_generator=wfg_mb,
+            priors=self.priors.copy()
+        )
+        self.assertAlmostEqual(likelihood1.reference_chirp_mass, likelihood2.reference_chirp_mass)
+
+    def test_no_reference_chirp_mass(self):
+        """
+        Check if AttributeError is raised if either reference_chirp_mass or chirp-mass prior is not specified
+        """
+        wfg_mb = bilby.gw.WaveformGenerator(
+            duration=self.duration, sampling_frequency=self.sampling_frequency,
+            frequency_domain_source_model=bilby.gw.source.binary_black_hole_frequency_sequence,
+            waveform_arguments=dict(
+                reference_frequency=self.fmin, approximant="IMRPhenomD"
+            )
+        )
+        with self.assertRaises(AttributeError):
+            bilby.gw.likelihood.MBGravitationalWaveTransient(
+                interferometers=self.ifos, waveform_generator=wfg_mb
+            )
+
 
 if __name__ == "__main__":
     unittest.main()