Add matrix type for spin/flavour symmetric matrices

[ffreyer]

I.e. a type for matrices of the form [A 0; 0 A]. We have this in both default Hubbard models in the hopping matrix.

I think this would be useful for measurements. Currently there is dispatch on the model type to differentiate these symmetric matrices from full matrices. But in general multiple model could have this symmetry and require the same specialized method. If we had a matrix type to dispatch on we could treat this more generally.

[ffreyer]

I added a Value type holding the number of flavors for dispatch in measurements instead. Not sure if that's a good solution

[ffreyer]

I added DiagonallyRepeatingMatrix in #165 but it's only used in measurements atm