Adding PowerModels "var" lookup

[ccoffrin]

Turns all JuMP variables into anonymous variables.

[codecov]

Codecov Report

Merging #149 into master will decrease coverage by 0.01%.
The diff coverage is 87.8%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
- Coverage      92%   91.98%   -0.02%     
==========================================
  Files          32       33       +1     
  Lines        3015     2996      -19     
==========================================
- Hits         2774     2756      -18     
+ Misses        241      240       -1

Impacted Files	Coverage Δ
src/core/base.jl	88.59% <ø> (-0.2%)	⬇️
src/prob/pf.jl	100% <100%> (ø)	⬆️
src/core/constraint_template.jl	85.71% <100%> (-1.62%)	⬇️
src/form/act.jl	100% <100%> (ø)	⬆️
src/form/wr.jl	99.49% <100%> (+1.01%)	⬆️
src/core/variable.jl	100% <100%> (ø)	⬆️
src/form/wrm.jl	100% <100%> (ø)	⬆️
src/prob/misc.jl	100% <100%> (ø)	⬆️
src/core/solution.jl	87.71% <100%> (ø)	⬆️
src/form/shared.jl	94.11% <100%> (+0.78%)	⬆️
... and 13 more

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