common constant function

[matbesancon]

Merge MOI._constant and MOIU.getconstant. Fixes #618

[blegat]

We might also add a way to get zero for the constant of SingleVariable and VectorOfVariables to solve the use case in:
https://github.com/JuliaOpt/MathOptInterface.jl/blob/c1d8f6a6949a39091c89d8c4053d51a7f6cc8d03/src/Bridges/scalarizebridge.jl#L7-L10
For that we need to know what is the coefficient type. We could allow a second argument which is the coefficient type but is optional for every set/function except SingleVariable/VectorOfVariables.

For the name, getconstant does not satisfy JuMP style guide, it should be constant, get_constant, or set_constant/function_constant. The second choice is inconsistent with the naming in the rest of JuMP/MOI but the first and third choices works (the first one is better IMO since the fact that it is a setorfunction` is already in the signature so adding it in the name is redundant).

[codecov-io]

Codecov Report

Merging #723 into master will not change coverage.
The diff coverage is 84.61%.

@@           Coverage Diff           @@
##           master     #723   +/-   ##
=======================================
  Coverage   93.83%   93.83%           
=======================================
  Files          54       54           
  Lines        5677     5677           
=======================================
  Hits         5327     5327           
  Misses        350      350

Impacted Files	Coverage Δ
src/Bridges/quadtosocbridge.jl	93.44% <100%> (ø)	⬆️
src/sets.jl	94.11% <100%> (+0.36%)	⬆️
src/Bridges/vectorizebridge.jl	85.71% <100%> (ø)	⬆️
src/Utilities/sets.jl	100% <100%> (ø)	⬆️
src/Bridges/scalarizebridge.jl	53.12% <50%> (+0.34%)	⬆️
src/functions.jl	95.16% <77.77%> (-3.12%)	⬇️
src/Utilities/functions.jl	91.86% <83.33%> (ø)	⬆️

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[blegat]

constant could appear somewhere in the doc, e.g. after http://www.juliaopt.org/MathOptInterface.jl/dev/apireference/#MathOptInterface.output_dimension
and http://www.juliaopt.org/MathOptInterface.jl/dev/apireference/#MathOptInterface.dimension

[matbesancon]

I put it after output_dimension

[matbesancon]

ok this looks good

[matbesancon]

@blegat seems good here