[FEATURE] Composite Block for GTFS

[rhazn]

User Story

1. As a {user}
2. I want {to have a composite block for handling GTFS data}
3. So that {I don't have to set up a very complex pipeline every time}

User Acceptance Criteria

• Basic functionality for composite blocks Composite blocktypes: Basic functionality #411
• model validation hints concerning composite blocks Composite blocktypes: Basic functionality #411
• tests Added validation tests for composite blocktypes #440
• docs Added documentation for composite blocks #438
• add a GTFS/CSV composite block into the stdlib Added CSVExtractor and GTFSExtractor to stdlib #437
• Multiple pipelines should be possible, moved to [FEATURE] Composite blocks should support the same pipeline syntax as pipelines #439

Examples

• https://github.com/jvalue/amse-exercises/blob/main/exercise5.jv

Definitions of Done

• A PR has been opened and accepted
• All user acceptance criteria are met
• All tests are passing