Add a minimal matrix object implementation

[wucas]

This PR adds a matrix object IsMinimalMatrixRep. Internally it represents the matrix as a flat list in row-major form. It only the required methods.

This is intended to be used as a minimal example and in order to test code, in particular the generic methods.

Regarding tests, if nothing obvious is wrong I'd like to merge this asap and then use it with the new test suite. Any errors could be fixed then.

This resolves Issue #5142.

[wucas]

Feedback on whether or not to have this in the release notes is welcome.

[ChrisJefferson]

This code seems in a basically good state. Obviously tests for it, and using it, are important but can be part of future work.

Only one thought I have now, before this is merged -- given that most people don't read manuals, I worry that people might see IsMinimalMatrixRep and think it is small, or minimal, and start using it. At the moment, that wouldn't be a completely stupid choice, as it does compress the matrix quite well.

As it might get slower (and less useful) as tests are added to it, I might give it a more explicit name, like IsExampleMinimalMatrixRep (other names OK of course, just something to make it clear people shouldn't be using it).

[wucas]

@ChrisJefferson Thanks for the suggestion. I didn't think of that way to understand the name but it is sensible to make it absolutely clear in the name that it is not intended for usage in computations. I chose the name IsMinimalExampleMatrixRep.

[fingolfin]

Hint: if you replace This takes care of Issue #5142. by This resolves #5142. then it will automatically close that issue when this PR is closed (we can also activate this by manually "linking" the issue and PR, but I think it's good to know about the possibility to activate this with keywords, it's IMHO quite useful.