Bridges should return get by value, not reference

[odow]

Check all get return values not references that the user can modify without fear.

As one example:

MathOptInterface.jl/src/Bridges/Constraint/geomean.jl

Line 173 in 7923c60

MOI.get(b::GeoMeanBridge, ::MOI.ListOfVariableIndices) = b.xij

If the user were to modify the returned value, this would modify the vector inside the bridge.

The fix would be

MOI.get(b::GeoMeanBridge, ::MOI.ListOfVariableIndices) = copy(b.xij)