add DualGeometricMeanCone and bridge

[araujoms]

Part of #3037

I've added the set, related tests, and the bridge. Still missing are the bridge tests; I'm submitting the PR prematurely because it's already getting big and I'm getting nervous about losing stuff.

I also didn't add support in the file format, I don't know if that's desired.

Basic

• Add a new AbstractScalarSet or AbstractVectorSet to src/sets.jl
• If isbitstype(S) == false, implement Base.copy(set::S)
• If isbitstype(S) == false, implement Base.:(==)(x::S, y::S)
• If an AbstractVectorSet, implement dimension(set::S), unless the
  dimension is given by set.dimension.
• Ensure the set does not contain references to any variables or constraints

Utilities

• If an AbstractVectorSet, implement Utilities.set_dot,
  unless the dot product between two vectors in the set is equivalent to
  LinearAlgebra.dot
• If an AbstractVectorSet, implement Utilities.set_with_dimension in
  src/Utilities/matrix_of_constraints.jl
• Add the set to the @model macro at the bottom of src/Utilities.model.jl

Documentation

• Add a docstring, which gives the mathematical definition of the set,
  along with an ## Example block containing a jldoctest
• Add the docstring to docs/src/reference/standard_form.md
• Add the set to the relevant table in docs/src/manual/standard_form.md

Tests

• Define a new _set(::Type{S}) method in src/Test/test_basic_constraint.jl
  and add the name of the set to the list at the bottom of that files
• If the set has any checks in its constructor, add tests to test/sets.jl

MathOptFormat

• Open an issue at https://github.com/jump-dev/MathOptFormat to add
  support for the new set New sets MathOptFormat#37

Optional

• Implement dual_set(::S) and dual_set_type(::Type{S})
• Add new tests to the Test submodule exercising your new set
• Add new bridges to convert your set into more commonly used sets

[odow]

I guess you see now why we've only added sets when they became needed. It's a lot of work!

[araujoms]

Indeed. I intend to add the dual vector relative entropy cone as well, but I don't use the other cones so I'll leave them for when they're needed.