Docu

• add custom component and Gro.data docu
• update getting_started/components.md with component/implementation phrasing +
• new structure -> component: super class, implementation: module with function and class; in getting_started/components.md and notebook 4 (eventually simplified in notebook 4)
• add docu for existing custom components in getting_started/components.md
• explain usage and editing of main/example.py
• add paragraph on writing Integration tests for new component