GDALSource WCS support

[SoerenHoffstedt]

GDAL 2.3 build steps:
First uninstall all gdal packages. To find installed gdal packages: apt list --installed | grep gdal.
Additionally to the gdal package from ubuntu-gis I had libgdal20 and gdal-data (pre-)installed.

wget http://download.osgeo.org/gdal/2.3.0/gdal-2.3.0.tar.gz
tar -xvzf gdal-2.3.0.tar.gz
cd gdal-2.3.0
./configure
make
sudo make install

Changes:

• Adjusting path of some #include directives and changing the linking of GDAL in CMake because of self compiled GDAL 2.3
• GDALSource: Skipping time snapping if the time placeholder string is not present in the filename.
• GDALSource: If path of dataset is empty, it is ignored when assembling the filename for the LoadingInfo. This way WCS can be used.
• GDALSource: If a WCS request is detected, open options are added to force NoGridAxisSwap for the GDAL WCS driver. Without this option the requests will always fail because the driver is swapping axes. Seems to be a bug a in the driver.
• Requesting only the wanted raster data from WCS is already happening through the driver during the RasterIO call. But the rectangle the driver is requesting seems to be buggy, because it always uses the lower bound of the dimensions instead of the wanted coordinate (e.g. -180 even though -50 is requested)
• GDALSource test queries for WCS requests and request without snapping

[SoerenHoffstedt]

I saw that GDAL 2.3.1 was released. The changes to the WCS driver are not related to our issues. Because of long compile times I couldn't already check if the 2.3.1 release works without problems.