Add gammalib::filepath method.

The gammalib::filepath combines a path name and a file name into a string. If the path name is empty, the file name is returned.
Changed logic of GCaldb class.

Now, GCaldb only reads the CALDB environment variable when needed. This allows changing the CALDB environment variable during runtime.

Rename GCaldb::dir to GCaldb::rootdir.
Merge branch '1102-cleanup-caldb' into integration

Conflicts:
	ChangeLog
	NEWS