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

[odow]

Closes #730

This needs discussion in our call:

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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%)	⬆️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 685fbb2...8cd110c. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.