ENH add --data option to get command, matching push one

[bpinsard]

Partially addresses #7468 by adding a --data option to get command with values similar to push command.
Passes --want-get here to git-annex get if auto or auto-if-wanted (and preferred content is set) values are passed.

PR checklist

• Write tests.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.93% 🎉

Comparison is base (eeb7313) 90.61% compared to head (4e4344f) 91.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7476      +/-   ##
==========================================
+ Coverage   90.61%   91.55%   +0.93%     
==========================================
  Files         325      325              
  Lines       43405    43449      +44     
  Branches        0     5826    +5826     
==========================================
+ Hits        39333    39781     +448     
+ Misses       4072     3653     -419     
- Partials        0       15      +15     

Files Changed	Coverage Δ
datalad/distribution/get.py	96.19% <100.00%> (+0.83%)	⬆️
datalad/distribution/tests/test_get.py	100.00% <100.00%> (ø)

... and 73 files with indirect coverage changes

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[yarikoptic]

I feel that introduction of data potentially makes get_data obsolete and thus require replacement/deprecation cycle and that could be taken as a good thing since get_data flag semantic is already shown to be "inconsistent" (get_data in python but --no-data in CLI).

But -n (AKA --no-data) is used heavily in handbook etc, and is a convenient shortcut to signal --data=nothing.

So, I think that is how -n|--no-data should be promoted - that it is a shortcut for data="nothing" and there should be explicit statement that data option is ignored if --no-data is provided.

But also let's wait for others to chime in

[codeclimate]

Code Climate has analyzed commit 4e4344f and detected 0 issues on this pull request.

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[adswa]

I have placed this PR on the agenda of the next dev call for October 24th, hopefully we can revive the discussion. :)