Extended unit tests/CI tests

[piberger]

Current CI test running on travis is checking eg.:

• simple skimming with multiple cuts and output files with SampleTree class (as done in the caching steps)
• adding branches (as in sys step)
• callbacks in SampleTree class (as needed for sys step)
• automatic enabling/disabling of used/unused branches
  One could add:
• create simple dummy datacards