Add support for broadcast with unstable return type

[blegat]

Also added a small comment explaining how broadcasting is implemented

Closes #1673

[codecov]

Codecov Report

Merging #1674 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1674      +/-   ##
==========================================
- Coverage   67.91%   67.89%   -0.02%     
==========================================
  Files          30       30              
  Lines        3805     3806       +1     
==========================================
  Hits         2584     2584              
- Misses       1221     1222       +1

Impacted Files	Coverage Δ
src/Containers/SparseAxisArray.jl	64.1% <0%> (-0.84%)	⬇️

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[codecov]

Codecov Report

Merging #1674 into master will increase coverage by 0.57%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1674      +/-   ##
==========================================
+ Coverage   67.91%   68.48%   +0.57%     
==========================================
  Files          30       30              
  Lines        3805     3900      +95     
==========================================
+ Hits         2584     2671      +87     
- Misses       1221     1229       +8

Impacted Files	Coverage Δ
src/Containers/SparseAxisArray.jl	64.1% <0%> (-0.84%)	⬇️
src/aff_expr.jl	72.13% <0%> (+0.57%)	⬆️
src/quad_expr.jl	71.73% <0%> (+1.15%)	⬆️
src/objective.jl	94.73% <0%> (+1.4%)	⬆️
src/variables.jl	86.26% <0%> (+4.05%)	⬆️

---

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