`VariablePrimal(N)` maybe should be `result_index` instead of `N`

[Wikunia]

I think it would be cleaner to have result_index as the field name as it is the case for ObjectiveValue.
Currently it is N see:
https://github.com/JuliaOpt/MathOptInterface.jl/blob/master/src/attributes.jl#L941-L951

Additionally in objective value it is referred to ObjectiveValue(resultidx) why not use either resultidx or result_index for both? I know this will break things but I think it would make it easier to read.

[odow]

Closing as duplicate of #960