Allow creating model from parts

[blegat]

We allow the users to create the model without using the MOI interface which might seem scary. But this will probably only be used either by advanced users or using something like https://github.com/jump-dev/MatrixOptInterface.jl/pull/35

Needed for https://github.com/jump-dev/MatrixOptInterface.jl/pull/35

[odow]

Tests please

[blegat]

I'll first make https://github.com/jump-dev/MatrixOptInterface.jl/pull/35 and then take a miniature version of https://github.com/jump-dev/MatrixOptInterface.jl/pull/35 as test here

[blegat]

I don't understand the codecov missing lines, it's reporting the signature of function as not covered but then its body is covered...