New submodule observables
- Created a new submodule prodimopy.observables, which should contain code that deals with synthetic observables
- moved the casasim routines to that submodule
- moved the FLitS stuff to that module
- added some initial stuff for JWST (Claude)
Those changes are not backwards compatible, but one only has to change some import statements.
- also improved some tests, including running notebooks as a testcase)