Collection of tasks

[moritzlayer]

• clean-up all functions
• write doc-strings for sphinx
• move functions where they belong
• refactor check_and_store
• replace pint wrappers
• check saving of sensitivity_measure
• check return values of _single functions in network (with or without [0])
• _effective_connectivity requires omega but doesnt use it

[moritzlayer]

• is transfer_function_shift missing the part coming from the variation of the variance?

[moritzlayer]

• Isn't V_0_rel always 0?