Add cli option to include an override.nix file

[seppeljordan]

Users can now specify -O file://path/to/file to include an
additional overrides file.

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This change is 

[seppeljordan]

I would appreciate suggestions on how to test this feature automatically.

[seppeljordan]

@garbas
I feel confident about the changes in this PR but I don't want to merge it yet. Could you look into this to see if it goes in a direction you approve?

[garbas]

💯 for the new option. i haven't tested it but an example which tests this would is required when we introduce new functionality.

you can change an already existing example to use this -O option.

[seppeljordan]

@garbas will do

[seppeljordan]

@garbas for some reason, nix-prefech-git is not in path anymore and i don't know why. locally the tornado test passes.

[garbas]

@seppeljordan I think we need to fix this in default.nix and depend directly on nix-prefetch-git and nix-prefetch-hg instead of nix-prefetch-scripts.

[seppeljordan]

okay, i solved the path issue in default.nix with wrapProgram --prefix PATH : /path/to/nix-prefetch-git but now we have a problem with scipy. i don't think that its an issue with this PR. The new connexion test makes use of the new feature. IMO we can merge this know. How do you feel about this, @garbas.