started removing FilebasedImageSource again
- removed FilebasedImageSource

- added more getter functions to ImageSource interface and implemented
them in the different ImageSources
Reworked the concept of the landmark loading:

- Renamed LandmarkFileLoader to LandmarkFileGatherer and changed its
interface to now be responsible for just gathering the files from
different sources.
- The LandmarkFileParsers now take a file as input
- Extended ImageSource interface

Full functionality not yet implemented
Implemented GatherMethod::ONE_FILE_PER_IMAGE_SAME_DIR
Implemented GatherMethod::ONE_FILE_PER_IMAGE_DIFFERENT_DIRS
Declared some of the new ImageSource functions as const
Implemented GatherMethod::SEPARATE_FILES
Removed some Todo's, updated documentation

- fixed missing initialization of index variable
Fixed some Todo's in the ImageSources regarding next(), get() and getImage().