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.

bilby/gw/likelihood/multiband.py

@@ -21,8 +21,9 @@ class MBGravitationalWaveTransient(GravitationalWaveTransient):
         A list of `bilby.detector.Interferometer` instances - contains the detector data and power spectral densities
     waveform_generator: `bilby.waveform_generator.WaveformGenerator`
         An object which computes the frequency-domain strain of the signal, given some set of parameters
-    reference_chirp_mass: float
-        A reference chirp mass for determining the frequency banding
+    reference_chirp_mass: float, optional
+        A reference chirp mass for determining the frequency banding. This is set to prior minimum of chirp mass if
+        not specified. Hence this option or prior containing the key `chirp_mass` is required.
     highest_mode: int, optional
         The maximum magnetic number of gravitational-wave moments. Default is 2
     linear_interpolation: bool, optional
@@ -72,10 +73,11 @@ class MBGravitationalWaveTransient(GravitationalWaveTransient):
 
     """
     def __init__(
-            self, interferometers, waveform_generator, reference_chirp_mass, highest_mode=2, linear_interpolation=True,
-            accuracy_factor=5, time_offset=None, delta_f_end=None, maximum_banding_frequency=None,
-            minimum_banding_duration=0., distance_marginalization=False, phase_marginalization=False, priors=None,
-            distance_marginalization_lookup_table=None, reference_frame="sky", time_reference="geocenter"
+            self, interferometers, waveform_generator, reference_chirp_mass=None, highest_mode=2,
+            linear_interpolation=True, accuracy_factor=5, time_offset=None, delta_f_end=None,
+            maximum_banding_frequency=None, minimum_banding_duration=0., distance_marginalization=False,
+            phase_marginalization=False, priors=None, distance_marginalization_lookup_table=None,
+            reference_frame="sky", time_reference="geocenter"
     ):
         super(MBGravitationalWaveTransient, self).__init__(
             interferometers=interferometers, waveform_generator=waveform_generator, priors=priors,
@@ -108,7 +110,21 @@ class MBGravitationalWaveTransient(GravitationalWaveTransient):
         if isinstance(reference_chirp_mass, int) or isinstance(reference_chirp_mass, float):
             self._reference_chirp_mass = reference_chirp_mass
         else:
-            raise TypeError("reference_chirp_mass must be a number")
+            logger.warning(
+                "No int or float number has been passed to reference_chirp_mass. "
+                "Checking prior minimum of chirp mass ..."
+            )
+            if self.priors is not None and "chirp_mass" in self.priors:
+                self._reference_chirp_mass = self.priors["chirp_mass"].minimum
+                logger.warning(
+                    "reference_chirp_mass is automatically set to prior minimum of chirp mass: "
+                    f"{self.priors['chirp_mass'].minimum}."
+                )
+            else:
+                raise AttributeError(
+                    "prior containing `chirp_mass` is required when no int or float number has been passed to"
+                    " reference_chirp_mass."
+                )
 
     @property
     def highest_mode(self):