Improved structure and added documentation (v2.0.0b1

• Improved structure of eta_x.envs
• Improved structure of eta_x.common
• Added examples
• Added testing for examples
• Significantly extended documentation
• Fixed typing problems in eta_x
• Extracted ETAx functionality into the eta_x.common module
• Removed fixed logging verbosity values for parts of the library
• Add automatic documentation for the entire API
• Automate version with git tags