(Un)Register a callback

        Parameters
        ----------
        callback: method handle
            Method to be registered or unregistered.
        remove=False: bool
            Whether to unregister the callback.

def register_callback(self, callback, remove=False):
        """(Un)Register a callback

        Parameters
        ----------
        callback: method handle
            Method to be registered or unregistered.
        remove=False: bool
            Whether to unregister the callback."""

        # (Un)Register the callback.
        if remove and callback in self.callbacks:
            self.callbacks.remove(callback)
        elif not remove and callback not in self.callbacks:
            self.callbacks.append(callback)