Using ProximalOperators.SlicedSeperableSum instead of ConeProduct

[mfalt]

Removes built in ConeProduct in favor of ProximalOperators SlicedSeparableSum, should benchmark before pull

[codecov-io]

Codecov Report

Merging #2 into master will increase coverage by 2%.
The diff coverage is 94.44%.

@@           Coverage Diff           @@
##           master       #2   +/-   ##
=======================================
+ Coverage   57.01%   59.01%   +2%     
=======================================
  Files          20       20           
  Lines         649      632   -17     
=======================================
+ Hits          370      373    +3     
+ Misses        279      259   -20

Impacted Files	Coverage Δ
src/types.jl	80% <100%> (ø)	⬆️
src/FOSSolverInterface.jl	92.1% <100%> (ø)	⬆️
src/cones.jl	94.44% <90.9%> (+35.95%)	⬆️

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[coveralls]

Coverage increased (+2.008%) to 59.019% when pulling 2fc51a9 on coneproduct into 6f18c58 on master.