Fix standard_form

[joaquimg]

@blegat is this (part of) the solution for #283 ?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.20%. Comparing base (bf2c4c3) to head (2ce98f3).

Files with missing lines	Patch %	Lines
src/utils.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #291   +/-   ##
=======================================
  Coverage   89.20%   89.20%           
=======================================
  Files          15       15           
  Lines        1973     1973           
=======================================
  Hits         1760     1760           
  Misses        213      213           

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[joaquimg]

JuMP.coefficient does not go well with vector affine functions because in the JuMP level they are a Vector of AffExpr, hence there is no JuMP.coefficient(::SomeFunction, ::Integer, ::VariableRef).
Possibly the more JuMP-like API is allowing getindex to add a lazy layer on top of MatrixVectorAffineFunction, but that might be too complex to maintain.