gmcat: various changes

1. Added some verbosity in the default mode to better monitor the progress of execution

The current version of gmcat when run without the -v command-line argument does not produce any output, so it's difficult to understand whether it is running or hanging. This change added a little verbosity, e.g. one line per loaded plugin, to allow the users to monitor the execution progress, especially when running gmcat interactively. It also informs the user about the location of the verbose output log.

2. Added a new command-line flag for passing the root directory of the local GeoModelATLAS repository

The current version of gmcat assumes it is run at some location inside the local GeoModelATLAS git repository. This poses some inconveniences to the users (like myself) who prefer to build the libraries outside the source tree and to run the applications outside of the source tree too. This change introduced a new -g command-line argument for gmcat allowing the users to pass the path to the local GeoModelATLAS repository.