Alpha Release 0.0.18-alpha

What's Changed

• Docs: Update Landingpage by @georg-schwarz in #424
• Fixed constraint default property value retrieval by @f3l1x98 in #423
• Feature ConstraintExecutor tests by @f3l1x98 in #428
• Fix default property retrieval by @georg-schwarz in #429
• Added design principles by @rhazn in #434
• Composite blocktypes: Basic functionality by @rhazn in #411
• Feature/396 handle xlsx files by @A-M-A-X in #433
• Added validation tests for composite blocktypes by @rhazn in #440
• Added CSVExtractor and GTFSExtractor to stdlib by @rhazn in #437
• Added documentation for composite blocks by @rhazn in #438
• Testing documentation by @f3l1x98 in #414
• Add open source notices and update Sheetjs version by @rhazn in #441
• Document Constraint Types in Code by @georg-schwarz in #444
• runPipeline returns ExitCode.FAILURE on error by @georg-schwarz in #448

Full Changelog: v0.0.17-alpha...v0.0.18-alpha