[FEATURE] create SwathWindowLoader

- allow loading of Swath windows file
- move code from OpenSwathWorkflow
[TEST] Test SwathWindowLoader
[DOC] doc, comments
[INTERNAL] rename variable, function signature
[FIX] cleanup, add Chris suggestions
[TEST] also test the exceptions
Merge pull request #1118 from hroest/feature/SwathWindowLoader

Feature/swath window loader