Suggestion: use absolute imports in app code to allow re-use

[ml-evs]

As you can't import the app code, it could be nice to switch to absolute imports for the optimade.server imports in e.g. main.py so that the file can be easily copied and dropped into other projects, without having to fork optimade-python-tools. This will also make it easier to update to later versions of python-tools too (assuming the app code itself doesn't change much).

e.g.

optimade-python-tools/optimade/server/main.py

Lines 16 to 20 in 8651cd5

	from .entry_collections import MongoCollection
	from .config import CONFIG
	from .middleware import EnsureQueryParamIntegrity
	from .routers import info, links, references, structures, landing
	from .routers.utils import get_providers, BASE_URL_PREFIXES

would become from optimade.server.entry_collections import MongoCollection.

Thoughts? Can add good-first-issue tag to this if we think its a good idea.

[CasperWA]

This is a great idea. We should use absolute imports all over. I agree.

[ml-evs]

I think we've done this everywhere I had in mind, so this was probably closed by #380.

[CasperWA]

There are still some cases in the models module. I'll quickly create a PR for this.