rewrite schedulers

[arnaud-ma]

I tried to implement #114 and I quickly got lost in all these if/else branches (surely added bit by bit). I noticed that the problem was that everything was mixed up between what is chunking and what is not. And everything that is chunking is common to each scheduler.

So I rewrote the module to make the difference between the two clear. This is done by encapsulating everything related to chunking into a new internal type ChunkingArgs. Then each Scheduler contains only one instance of this type instead of all the chunking arguments.

Otherwise nothing should really change outside the module, except:

• shed(;chunking=false, split=:whatever) returned an ArgumentError. It's no longer the case. This makes the code simpler and I think it fits better with what the documentation describes: "For chunking=false, the arguments nchunks/ntasks, chunksize, and split are ignored"
• Using sched.nchunks, sched.chunkisze or sched.split no longer works and is replaced by functions of the same name (e.g. nchunks(sched). This is because they are encapsulated into sched.chunking_args, so they are no longer direct fields. We could implement a Base.getproperty though, but I don't think it's really necessary.

With these changes, adding chunking arguments or even a new type of Scheduler becomes easier.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 96.34146% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.28%. Comparing base (cba9127) to head (5b79e03).
Report is 132 commits behind head on master.

Files with missing lines	Patch %	Lines
src/schedulers.jl	97.50%	2 Missing ⚠️
src/implementation.jl	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
+ Coverage   90.24%   90.28%   +0.04%     
==========================================
  Files           3        7       +4     
  Lines          82      556     +474     
==========================================
+ Hits           74      502     +428     
- Misses          8       54      +46     

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