Saves TransitSignal.

        Calls :func:`TransitSignal.save`; default filename is
        ``trsig.pkl`` in ``self.folder``.

def save_signal(self,filename=None):
        """
        Saves TransitSignal.

        Calls :func:`TransitSignal.save`; default filename is
        ``trsig.pkl`` in ``self.folder``.
        """
        if filename is None:
            filename = os.path.join(self.folder,'trsig.pkl')
        self.trsig.save(filename)