Ycbvideo

- the availability check for 'frame sets' now checks for the existence of all corresponding files
- manual inspectation of the dataset on disk is now possible by executing `python -m ycbvideo`
- the package functionality is now secured by automatic tests
- the README contains exact instructions on how to use the package

- fixed imports
- package version is now maintained in VERSION file
- package version is also available through ycbvideo.__version__ at runtime