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 the prior minimum of chirp mass if
+        not specified. prior needs to be .
     highest_mode: int, optional
         The maximum magnetic number of gravitational-wave moments. Default is 2
     linear_interpolation: bool, optional
@@ -108,16 +109,22 @@ class MBGravitationalWaveTransient(GravitationalWaveTransient):
     def reference_chirp_mass(self, reference_chirp_mass):
         if isinstance(reference_chirp_mass, int) or isinstance(reference_chirp_mass, float):
             self._reference_chirp_mass = reference_chirp_mass
-        elif self.priors is not None and "chirp_mass" in self.priors:
-            self._reference_chirp_mass = self.priors["chirp_mass"].minimum
-            logger.warning(
-                "No number has been passed to reference_chirp_mass. It is automatically set to the prior minimum of"
-                f" chirp mass: {self.priors['chirp_mass'].minimum}."
-            )
         else:
-            raise AttributeError(
-                "A number needs to be passed to reference_chirp_mass or prior needs to include chirp mass."
+            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):