Make POT optional

[devmotion]

This PR extends https://github.com/zsteve/OptimalTransport.jl/pull/23 and makes POT optional. A submodule POT will be available (and exported) if users load PyCall.

This fixes https://github.com/zsteve/OptimalTransport.jl/issues/18.

[coveralls]

Pull Request Test Coverage Report for Build 418805948

• 8 of 8 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 51.152%

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Totals
Change from base Build 418497072:	0.2%
Covered Lines:	111
Relevant Lines:	217

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #29 (dfa1ccb) into master (618450e) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   50.92%   51.15%   +0.22%     
==========================================
  Files           2        3       +1     
  Lines         216      217       +1     
==========================================
+ Hits          110      111       +1     
  Misses        106      106              

Impacted Files	Coverage Δ
src/OptimalTransport.jl	45.07% <100.00%> (-1.66%)	⬇️
src/pot.jl	100.00% <100.00%> (ø)

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