GDAL dependency

[mrayson]

Try and import the GDAL-dependent maptools.py function but raise an exception if GDAL is not installed

[mrayson]

The way GDAL is imported needs updating... See here: https://pypi.org/project/GDAL/

"""

from osgeo import gdal
from osgeo import ogr
from osgeo import osr
from osgeo import gdal_array
from osgeo import gdalconst
"""