Convert constraint starting values

[blegat]

When writing this post, I found out when when starting integer dual starting values, COSMO was failing here:
https://github.com/oxfordcontrol/COSMO.jl/blob/dd6a6fdbda91231e4c3a108d072389e50a5003ac/src/interface.jl#L167
because y0 does not have the right element type.
As we are doing the conversion for variable starting values, I thought we might need to do it for constraints as well:

JuMP.jl/src/variables.jl

Line 1685 in b13812d

_convert_if_something(T, value),

Now, we might want to use MOI.Utilities.value_type as the value type of the function may not be the value type of the variables.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b13812d) 98.38% compared to head (c0053de) 98.20%.
Report is 1 commits behind head on master.

❗ Current head c0053de differs from pull request most recent head a6f15fe. Consider uploading reports for the commit a6f15fe to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3571      +/-   ##
==========================================
- Coverage   98.38%   98.20%   -0.18%     
==========================================
  Files          37       37              
  Lines        5624     5627       +3     
==========================================
- Hits         5533     5526       -7     
- Misses         91      101      +10     

Files	Coverage Δ
src/constraints.jl	96.85% <100.00%> (+0.05%)	⬆️

... and 4 files with indirect coverage changes

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[odow]

I simplified the implementation a little