Added new modules (calibration, peakfinding) and changed project structure

Contributions by Valerio Mariani:

 - Moved peakfinding and calibration algorithms from OnDA to CfelPyUtils
 - Added full type hinting
 - Updated documentation
 - Used more pythonic default values for some dictionary entries
 - Added mypy-extensions requirement
 - Added comment

Contributions by Florian Lauck:

 - Restructuring of project, subsequent refactoring and update of docs
 - Removed Python 2 code
 - Added unit tests, build tool, and continuous integration