Adds support to drop images

            n.generate_vrt(**kwargs)

    def to_isis(self, path, flistpath=None,sql = """

SELECT points.id, measures.serial, points.pointtype, points.apriori, points.adjusted,

measures.sample, measures.line, measures.measuretype, measures.imageid

FROM measures INNER JOIN points ON measures.pointid = points.id

WHERE points.active = True AND measures.active=TRUE AND measures.jigreject=FALSE;

SELECT points.id,

        measures.serial,

        points.pointtype, 

        points.apriori, 

        points.adjusted,

        measures.sample, 

        measures.line, 

        measures.measuretype,

        measures.imageid

FROM measures 

INNER JOIN points ON measures.pointid = points.id

WHERE 

    points.active = True AND 

    measures.active=TRUE AND 

    measures.jigreject=FALSE AND 

    measures.imageid NOT IN

        (SELECT measures.imageid

        FROM measures 

        INNER JOIN points ON measures.pointid = points.id

        WHERE measures.active = true and measures.jigreject = false AND points.active = True

        GROUP BY measures.imageid

        HAVING COUNT(DISTINCT measures.pointid)  < 3);

"""):

        """

        Given a set of points/measures in an autocnet database, generate an ISIS

            flistpath = os.path.splitext(path)[0] + '.lis'

        target = config['spatial'].get('target', None)

        ids = df['image_index'].unique()

        fpaths = [self.nodes[i]['data']['image_path'] for i in ids]

        for f in self.files:

            if f not in fpaths:

                warnings.warn(f'{f} in candidate graph but not in output network.')

        cnet.to_isis(df, path, targetname=target)

        cnet.write_filelist(self.files, path=flistpath)

        cnet.write_filelist(fpaths, path=flistpath)

    @staticmethod

    def update_from_jigsaw(session, path):