major summer patch

- version number increment
- **generic unit testing added** 
  (depending on exemplary calls of nodes, 
   automatic tests are generated)
- several customized unit tests added
- several minor corrections due to added tests
- improvement of the parameter replacement scheme
- contributor license agreement added
- improved credits
- largely improved the time for storing and loading node_chains
- added input and output type checks for nodes
- documentation and code style improvements (PEP8)
- several minor bugfixes