class SubCallButton(VCPButton):
    """Button for calling ngc subroutines.

    Args:
        parent (QWidget, optional) : The parent widget of the button, or None.
        filename (str, optional) : The filename of the NGCGUI style subroutine
            the button should call, including any extension. The subroutine must
            be on the subroutine path specified in the INI.