variable index not preserved in copy_to

[hannahtro]

• problem occurring in strong_branching.jl: the variable indices in the destination model do not match the source model
• the function should either be updated in MOI itself, see issue https://github.com/jump-dev/MathOptInterface.jl/issues/2236
• or we need our own copy_to function ensuring the identical variable indexing
• or we have to update f and grad! in Boscia to be linked to variable types and not just the indices