TarFiles should uniformly use TAR file names instead of paths

TarFiles#getIndices return full paths to identify the TAR files, while TarFiles#getGraph accepts TAR file names instead of full paths as an argument. This asymmetry in the API of TarFiles forces the user to convert between full paths and file names. Moreover, it leaks the location of the file store on disk, which is irrelevant to the users of TarFiles.