Implement MOI interface for LP forms

[blegat]

This PR reuses the LP structures and change_form functions that were present. However, the strategy from building a model from an LP structure is different. Instead of having a custom function that creates an MOIU model from an LP structure, the LP structures are now sub-types of MOI.ModelLike and they implement the read-only MOI API.
It also fills up some tests and the README to make it ready for users to start to play with it.
It still does not cover the use case of users wanting to extract a matrix form from a JuMP or MOI model but that is left as future work :)

[codecov]

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