Add bridge from Hermitian PSD to complex function in Symmetric PSD

[blegat]

See jump-dev/SeDuMi.jl#28 (comment)

Basic

• Create a new file in src/Bridges/XXX/bridges named after the type of
  the bridge
• Define the bridge, following existing examples. The name of the bridge
  struct must end in Bridge
• Check if your bridge can be a subtype of MOI.Bridges.Constraint.SetMapBridge
• Define a new const that is a SingleBridgeOptimizer wrapping the
  new bridge. The name of the const must be the name of the bridge, less
  the Bridge suffix
• If the bridge should be enabled by default, add the bridge to
  add_all_bridges at the bottom of src/Bridges/XXX/XXX.jl

Tests

• Create a new file in the appropriate subdirectory of tests/Bridges/XXX
  named after the type of the bridge
• Use MOI.Bridges.runtests to test various inputs and outputs of the
  bridge
• If, after opening the pull request to add the bridge, some lines are not
  covered by the tests, add additional bridge-specific tests to cover the
  untested lines.

Documentation

• Add a docstring which uses the same template as existing bridges.

Final touch

If the bridge depends on run-time values of other variables and constraints in
the model:

• Implement MOI.Utilities.needs_final_touch(::Bridge)
• Implement MOI.Utilities.final_touch(::Bridge, ::MOI.ModelLike)
• Ensure that final_touch can be called multiple times in a row

[odow]

What's the reason not to define the SingleBridgeOptimizer?

[odow]

I haven't looked in detail, but if you're happy I'm happy.