Features/restructure network

[p-snft]

Puts a clean structure to the submodules, i.e. components and blocks are now located next to each other (implements #588).

Summary:

• parts of the network are now clearly structured into buses, components, and flows.
• public and private API should be more distinguished now ('_' signifies private, public API should be exposed using __all__)
• started API for distinct Flow, InvestmentFlow, and NonConvexFlow
• experimental stuff is now sitting in submodules called experimental (replaces "custom")

Tasks:

• Restructure
• Adjust unit tests
• Adjust documentation (at least the part that is tested)
• Add new flow classes (NonConvexFlowand InvestmentFlow)
• Add to WhatsNew

[pep8speaks]

Hello @p-snft! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-11-15 10:03:42 UTC

[p-snft]

I am currently wondering if components, buses and flows should be available in solph directly or if categories should be provided. For example, do you from oemof.solph import GenericStorage or from oemof.solph.components import GenericStorage?

Little update: Currently, I allow both. Experimental stuff has to be imported from the specific submodule.

[p-snft]

I am not sure about the API of the InvestmentFlow, but as this might be added later, I will now convert this draft to a full PR.

[uvchik]

For me it looks fine, even though I may have missed something due to the high number of changes. But as it is not a merge into a stable branch we may have the chance to bring small issues in line, if there are any 😄