Allow to create derivation in different projects

[GianlucaFicarelli]

Until now, used and created entities were required to belong to the same project.

With this PR we allow to create new derivations from any readable entity (used) to a writable entity (generated).

• Used entity: a readable entity (public in any project, or private in the same project).
• Generated entity: a writable entity (public or private, in the same project).

Even when the parent (used) is private, the child (generated) can be either public or private.
Tests are added to test all the possible combinations.

Other related changes:

• The /hierarchy endpoint needed a fix to consider the case where a public generated entity G is derived from a private used entity U, and the user cannot read the private used entity U because it's in a different project. In that case, the public generated entity G should be returned as a root node, independently from the derivation type.
• The derived-from endpoint needed a fix to ensure that the specified generated entity is readable.

Fix #427

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
pytest	97.55% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
app/service/derivation.py	100.00% <100.00%> (ø)
app/service/hierarchy.py	100.00% <100.00%> (ø)

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[eleftherioszisis]

Makes sense, nice.