Move supports_default_copy_to to MOI?

[mlubin]

supports_default_copy_to is obligatory to implement according to http://www.juliaopt.org/MathOptInterface.jl/dev/apimanual/#Testing-guideline-1 and http://www.juliaopt.org/MathOptInterface.jl/dev/apimanual/#Implementing-copy-1. Should we move it from MOIU to MOI? MOIU should be for helper functions, not for core methods of the API that solvers need to implement.

[blegat]

default_copy_to is in MOI so it would reference something in MOIU from MOI but apart from that I don't see any reason not to move it to MOI

[mlubin]

We can move default_copy_to to MOI if it's part of the definition of the solver interface.