Add frequency mask setter

I have added a frequency mask setter for when cases when you want to specify the frequency mask manually so that it's something other than false outside the minimum and maximum frequencies.

bilby/gw/detector/strain_data.py

@@ -138,6 +138,11 @@ class InterferometerStrainData(object):
             self._frequency_mask_updated = True
         return self._frequency_mask
 
+    @frequency_mask.setter
+    def frequency_mask(self, mask):
+        self._frequency_mask = mask
+        self._frequency_mask_updated = True
+
     @property
     def alpha(self):
         return 2 * self.roll_off / self.duration