VariablePrimalStart doesn't support incremental setting

[odow]

LQOI allows setting VariablePrimalStart one-by-one:
https://github.com/JuliaOpt/LinQuadOptInterface.jl/blob/148334e6b74c37f2108ff8dc92169d18e99584a7/src/variables.jl#L204-L207
But Gurobi does not cache the start values and tries to set the basis every time:
https://github.com/JuliaOpt/Gurobi.jl/blob/bd2e5073ea0e9d24accd47f51343683ce9f3b26a/src/MOI_wrapper.jl#L448-L458

This is fixed in the re-write of the Gurobi wrapper, but it needs a test.

[odow]

Closed by #216