Schema of return value of "callbacks"

[soxofaan]

We have a couple of processes that take a "callback" (I know we deprecated that term, but using it avoids confusion here I think), e.g.:

• reduce_dimension has parameter "reducer": a callback that takes a data argument (type: array of any)
• apply has parameter "process": a callback that takes a x argument (type: any)
• apply_neighborhood has parameter "process": a callback that takes a data argument (type: rastercube)

Maybe this has been discussed before, but is there any reason we don't define the schema/type of what the callback should return?
For example:

• for reduce_dimension: (array[any]) -> any
• for apply: (any) -> any
• for apply_neighborhood: (rastercube) -> rastercube
• for count: (any) -> bool

[m-mohr]

Wow! That is indeed missing and never came up before, but is so integral. We need to add that for sure!