Set the capstyle as a string in ('butt', 'round', 'projecting')

def set_capstyle(self, cs):
        """
        Set the capstyle as a string in ('butt', 'round', 'projecting')
        """
        DEBUG_MSG("set_capstyle()", 1, self)
        self.select()
        GraphicsContextBase.set_capstyle(self, cs)
        self._pen.SetCap(GraphicsContextWx._capd[self._capstyle])
        self.gfx_ctx.SetPen(self._pen)
        self.unselect()