Implement operate: Vector + and / constant

[blegat]

See https://github.com/JuliaOpt/MathOptInterface.jl/pull/818/files#r314057544

[codecov-io]

Codecov Report

Merging #837 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #837      +/-   ##
==========================================
+ Coverage   94.83%   94.83%   +<.01%     
==========================================
  Files          71       71              
  Lines        7643     7649       +6     
==========================================
+ Hits         7248     7254       +6     
  Misses        395      395

Impacted Files	Coverage Δ
src/Utilities/functions.jl	94.59% <100%> (+0.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bb800e9...a8a2c76. Read the comment docs.

[chriscoey]

Thanks!