general refactorings, changes and code cleanup

[VictorVerhaert]

small changes before publishing to pypi as discussed in a meeting with @VincentVerelst @GriffinBabe

• [x] load_collection -> _load_collection
• [ ] commons.py -> _commons.py
• [x] cldmask -> cloudmask
• [x] fetcher and processors for S1 and S2 (e.g. get_s2_l2a_default_fetcher -> _get_s2_l2a_default_fetcher)
• [x] remove multitemporal_speckle()
• [x] add most used functions to te relevant init files (e.g. extractors, job splitters, compositing methods)
• [ ] add annotations like deprecated and experimental where appropriate