CRUD Logic for entrypoint and InputOutput

[mottegk]

Based on #36
As the Entrypoint does not need to be set at the time a block is added to a project, and we have no other way to set it for a block, the DAG-translator currently does not work when Entrypoint is not set from the beginning. As this is unintended behaviour we need another option to add a selected entrypoint to a block in a project.

[PaulKalho]

This is part of #69. Regarding my exams, I will investigate how we could close #69 as soon as possible and split up the rest of the tasks.

Entrypoints and InputOutputs will only be created internally. The user will not have to take care of this.

[mottegk]

Ahh, thanks for pointing that out to me!
Good luck on your exams 🍀