[WIP][RF] Use lazy loading

[skoudoro]

Proof of concept for adopting Scientific Python SPEC 1.

This has several advantages:

• It exposes a nested namespace that behaves as a flat namespace. This avoids carefully having to import exactly the right combination of submodules, and allows interactive exploration of the namespace in an interactive terminal.
• It avoids having to optimize for import cost. Currently, developers often move imports inside of functions to avoid slowing down importing their module. Lazy importing, when implemented through out a library, makes all imports cheap.
• It provides direct access to submodules, avoiding local namespace conflicts. Instead of doing import dipy.reconst as reconst to avoid clobbering a local reconst, one can now import each library and access its members directly: import dipy; dipy.reconst.

Todo

• Add CI run that uses EAGER_IMPORT=true env var to smoke out circular import issues
• Convert all __init__.py

Feedbacks are welcomed

When this PR will be done, a follow-up PR will be created to simplify all tutorials import