Disabling Chunking: unsupport nchunks=0 in favor of chunking=false

[carstenbauer]

As discussed with @MasonProtter in private, it isn't entirely obvious what nchunks=0 means - there are different possible interpretations - which is why we no longer support it. Instead, we introduce a new keyword argument chunking that can be used to toggle chunking.

Corresponding part of the docstring:

•  chunking::Bool (default true):
     • Controls whether input elements are grouped into chunks (true) or not (false).
     • For chunking=false, the arguments nchunks, chunksize, and split are ignored and input elements are
       regarded as "chunks" as is. Hence, there will be one parallel task spawned per input element. Note that,
       depending on the input, this might spawn many(!) tasks and can be costly!

Example:

julia> Threads.nthreads()
6

julia> tmapreduce(+, 1:12; scheduler=StaticScheduler(; chunking=false)) do i
           println(i, " → ", Threads.threadid())
           i
       end
3 → 3
4 → 4
9 → 3
10 → 4
2 → 2
6 → 6
8 → 2
5 → 5
12 → 6
11 → 5
1 → 1
7 → 1
78

[MasonProtter]

Yup, looks good!