Allow broadcasting in callbacks

[odow]

Raised on Discourse: https://discourse.julialang.org/t/jump-cplex-retrieve-value-of-array-variables-in-callback/54933/3

[codecov]

Codecov Report

Merging #354 (fa2ca04) into master (71e64ea) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #354      +/-   ##
==========================================
+ Coverage   41.96%   41.98%   +0.01%     
==========================================
  Files           8        8              
  Lines        3927     3928       +1     
==========================================
+ Hits         1648     1649       +1     
  Misses       2279     2279              

Impacted Files	Coverage Δ
src/MOI/MOI_callbacks.jl	92.66% <100.00%> (+0.06%)	⬆️

---

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 71e64ea...7ba333f. Read the comment docs.