feat: process links

[JakobBD]

Purpose of this PR

• Introduce a ProcessDefinition class, which allows setting the process ID explicitly. (Does not have to be used, strings are still okay)
• Introduce functions which add a link to inflows, outflows and a stock to the Process class

This created circular dependencies for pydantic's TYPE_CHECKING, which were treated with changes to init.

Requires #123 to #125, so they should be merged first

TODO (beyond the usual):

• implement a better process repr
• allow non-contiguous process IDs
• make sure the actual linking step during setup has survived the merge
• pull flexible_stock after their change to init

Type of change

(Make sure to delete from the Type-of-change list the items not relevant to your PR)

• Bug fix
• Refactoring
• New feature
• Minor change
• Major change

Checklist:

• I have updated the in-code documentation of all changed classes and functions
• I have added in-code documentation and type hints to all new classes and functions
• I have adapted the howtos
• I have adapted the examples
• If this change should entail a version update, I have bumped the version in the pyproject.toml file accordingly