[potentially breaking] Use JuMP.SkewSymmetricMatrixSpace in variable_bus_voltage(::AbstractWRMModel

[odow]

Closes https://github.com/lanl-ansi/PowerModels.jl/issues/730

This needs discussion in our call:

• What's up with the bounds?
• Is this breaking, because WI is now AffExpr?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.71%. Comparing base (685fbb2) to head (8cd110c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #968   +/-   ##
=======================================
  Coverage   93.71%   93.71%           
=======================================
  Files          43       43           
  Lines        9635     9638    +3     
=======================================
+ Hits         9029     9032    +3     
  Misses        606      606           

Files with missing lines	Coverage Δ
src/form/wrm.jl	96.73% <100.00%> (+0.03%)	:arrow_up:

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