Test GPS info on real camera trap images

[ivnpsvc]

Follow-up to #247 and #248.

Adds test_get_gps_info_real_images() to megadetector/data_management/read_exif.py, which runs
get_gps_info(), has_gps_info() and get_exif_lat_lon() on the real camera trap images in the new
gps-tests folder of the test data package, downloaded with md_tests.download_test_data(). It
covers valid GPS (including zero degrees latitude), null island (N/E and S/W refs, with and without
check_for_null_island), junk GPS values (reported as present, with no location), GPS tags without
lat/lon, an empty GPS IFD, no GPS, no EXIF, and a corrupt image, each as a filename, a PIL image, and
an EXIF dict. It also fails if an image is added to gps-tests without an expected result.

Everything the real images now cover is removed from the synthetic-image test, which keeps only the
cases that don't occur in real images: null island stored as NaN, a missing file, an exif_tags
field that is None, and string-valued GPS values.

[agentmorris]

Merged, thanks!