Merge branch 'feature-fixed_values' into 'develop'

Feature fixed values

See #126 

Order of prio of config values:
1. Fixed values
2. User config (-config / -cv on the command line)
3. Default files (coming from BIOPET_CONFIG env)
4. Default values inside the code.

Also changed way to use default values. No need for a merge step anymore, this happens inside the class now. This makes code inside pipelines/extensions cleaner.

Todo:
- [x] Basic implementation
- [x] Unit testing

See merge request !233