refactoring of element-wise updates

[hhaensel]

The current solution supplies the updated value and the vector of jskeys to the notify() routine.
This PR takes a more general approach and supplies a vector of pairs containing all objects (arrays or dicts) with their respective index. This makes it possible for users to write their own update functions that take into account, e.g. the index of the changed element.
Other changes:

• try-catch has been replaced by a proper methods() call to avoid decrease of performance.
• OffsetArrays are now handled correctly

This code could be transferred to Observables with only minor changes. I've commented an open issue to ask if they are interested.