Verify_segmentation.py: should flag ground truth data that contains lines or points

[remtav]

Although geo-deep-learning is meant to deal only with ground truth files containing polygons or multipolygons, no warning is issued if ground truth data contains such vector data.

The following ground truth data can be tested to see gdl's behavior with non-polygon ground truth: line_gt.zip

This zip contains a csv with dummy spacenet imagery