Add submodule `OptimalTransport.Dual` and examples for smoothed dual variational problems

[zsteve]

This PR adds a submodule Dual that provides the duals of the smoothed Wasserstein loss and their gradients (so far only entropy-regularised, but could expand to L2 or group-sparse regularisations, see https://arxiv.org/abs/1710.06276).

This kind of stuff is more geared towards variational problem use cases where OT is used as part of a loss function, rather than to 'statically' measure distances between prescribed distributions.

As an illustration of how dual approaches can be practically useful, the JKO example is updated with code that solves the dual problem (and this is much faster than using the primal approach of autodifferentiating sinkhorn).

An additional example implementing Wasserstein-NMF as described in Rolet et al. (https://proceedings.mlr.press/v51/rolet16.pdf) is provided, and this demonstrates uses of the Dual submodule.

P.S. apologies for the commits trying to get CIs to pass -- just figured out to set JULIA_LOAD_PATH="@" in order to prevent usage of packages from outside the active Julia environment!

[coveralls]

Pull Request Test Coverage Report for Build 1224285834

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 26 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-4.8%) to 94.424%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/utils.jl	0	5	0.0%
src/dual/entropic_dual.jl	0	21	0.0%
<!--	Total:	0	26	0.0%	-->

Totals
Change from base Build 1223996798:	-4.8%
Covered Lines:	508
Relevant Lines:	538

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💛 - Coveralls

[codecov-commenter]

Codecov Report

Merging #129 (7403b37) into master (f961027) will decrease coverage by 4.79%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
- Coverage   99.21%   94.42%   -4.80%     
==========================================
  Files          11       12       +1     
  Lines         512      538      +26     
==========================================
  Hits          508      508              
- Misses          4       30      +26     

Impacted Files	Coverage Δ
src/dual/entropic_dual.jl	0.00% <0.00%> (ø)
src/utils.jl	88.63% <0.00%> (-11.37%)	:arrow_down:

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[zsteve]

Let me know if anything further is needed for this PR! Regarding the function names, I guess "ot_entropic" could be shortened to "ot_ent". _grad could be replaced with a ∇ somewhere, but I think it would be good to continue the convention of avoiding unicode function names. In any case, these functions introduced are more "advanced" functionality, so perhaps longer function names is not so bad?

[zsteve]

Merging since there don't seem to be additional comments.