Move management of processes to document collection level

[reckart]

Describe the refactoring action Currently, methods for managing processes are exposed at the level of the project. However, processes are bound tightly to document collections, so it may be better to move the methods there.

Expected benefit Facilitate APi for the user e.g. os that an info such das the document collection name does not need to be specified multiple times:

• user creates a project -> gets a Project object
• user creates a document collection in the project -> gets a DocumentCollection object
• user creates a process on that document collection -> gets a Process object
• user fetches the results of the process -> gets e.g. a CAS or JSON object

[DavidHuebner]

As part of the ticket, I want to add a proper String representation of our objects (Client, Pipeline, Process, etc.). Right now, if we call print(Pipeline), we get something like: <averbis.core._rest_client.Pipeline object at 0x7f91657b7400>

Instead, I would like to have the output Pipeline(name="Discharge", project="test"). This can be archieved using the repr method for each class.