Change default behavior for `.datachain` directory

[shcheklein]

People keep hitting issues. pwd is too unpredictable, unstable, .datachain is created all over the place.

We should be creating it to the .git and / or a shared for the whole user by default.

[dmpetrov]

Is it about running dc from not a root dir of a repo? I'd appreciate more context?

[shcheklein]

Sure. By default DC creates .datachain in the directory you running it in (pwd). We end up having .datachain in many locations and they are all disconnected from each other.

Suggestion here is attach it to .git location or to some global location. There will less surprises. Now I recommend everyone to set DATACHAIN_ROOT in their shell config to make it usable.