Support GNAT project files

[Jellix]

We are already using GNATCOLL and the files we parse are rather GNAT specific, so to be in line with other tools, we could just as well use a .gpr file as input instead of wildly scanning directories. This is especially important as specifying the object directory to parse also catches files from the saved_runs subdirectory, causing inconsistent behaviour by including data from earlier runs. Parsing the project files and selectively finding the associated .spark should be more user friendly.