Make GDAL dependency optional

[kautlenbachs]

It is needed only for deeper analysis where intensity file could be saved as a geotiff for example.
For environments where only initial goal is intended - produce only envisat formats - this is not needed.

This could be implemented as (to separate/segregate debug/analysis):

• In the CMakeLists.txt file check if GDAL is present
  • If it is set a define e.g. GDAL_ENABLED
• Based on the defined symbol link gdal and inside the main flow use it to enable secret 'intensity' option to write TIFF otherwise log a warning
• Also GDAL specific error checks move into single file e.g. img_output.cc

for ootpa runtime container installation, remove GDAL e.g. update setup_runtime.sh script. Add GDAL to development setups then.