Reorganize sinkhorn barycenter

[zsteve]

This PR updates sinkhorn_barycenter to use the same conventions for argument and variable names as sinkhorn_gibbs and related functions.

[coveralls]

Pull Request Test Coverage Report for Build 890948598

• 31 of 36 (86.11%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 95.161%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/OptimalTransport.jl	31	36	86.11%
<!--	Total:	31	36	86.11%	-->

Totals
Change from base Build 889071712:	-0.7%
Covered Lines:	295
Relevant Lines:	310

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💛 - Coveralls

[codecov-commenter]

Codecov Report

Merging #89 (4dbe7e9) into master (ea089f2) will decrease coverage by 0.74%. The diff coverage is 86.11%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   95.90%   95.16%   -0.75%     
==========================================
  Files           1        1              
  Lines         293      310      +17     
==========================================
+ Hits          281      295      +14     
- Misses         12       15       +3     

Impacted Files	Coverage Δ
src/OptimalTransport.jl	95.16% <86.11%> (-0.75%)	:arrow_down:

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