Add interface to run datapackage and use hooks from outside django

[henhuy]

Django-oemof simulation shall be run from outside, including hooks which load data from pipe-datapackage.
Idea:

• Hooks can be included via sys.path.append
• Set path to datapackages (oemof and data from pipe datapacakge) in envs
• register hooks
• run simulation
• return results (instead of storing in DB) -> add flag to simulation function

In order to support this, django-oemof hooks must have as less dependencies as possible:

• import django.http.HttpRequest only when type checking
• set up path to datapackage not via django.settings? (or setup django without server?) Maybe try import from envs instead.
• use relative imports in hooks.py instead of absolute imports
• don't forward request to "Setup" hook -> form data has to be unpacked beforehand in this case