Various improvements to MatrixOfConstraints

[blegat]

Various improvements to MatrixOfConstraints. It can now pass the relevant tests in Test/modellike.jl.

• Add macros to create subtypes of StructOfConstraints and improve StructOfConstraints so that sub-fields can be MatrixOfConstraintss.
• [breaking] Use Box in MOIU.Model for storing variable bounds.
• Implement some of the getters for MOI attributes.

[odow]

What is the motivation for this?

[blegat]

What is the motivation for this?

The StructOfConstraints macro is needed, e.g. for solvers that have a separate matrix for equality constraints and conic constraints, e.g., ECOS.
Using Box in MOIU.Model allows to share code between MatrixOfConstraints and AbstractModel, notably the getter for AbstractSet which has a few methods.
For the getters, I get we could choose not to implement them but they are quite straightforward and if they are not supported, then the hacks we will need in the tests of the solver wrappers using a MatrixOfConstraints as cache would be more work than this PR (and maintaining these features). Moreover, having them allows to use more modellike tests like the one added in this PR which is quite useful to ensure that MatrixOfConstraints behaves consistenctly with how a good ModelLike should behave.

[odow]

These changes make sense, modulo some more documentation and testing.