fix braille and add small test

[VictorVanthilt]

This PR makes the braille function usable again for MPOHamiltonians, which are no longer considered SparseMPOs. Additionally it adds some hard-coded tests to ensure is keeps working in the future.

I had to add a new file src/operators/utility.jl which is included after all the other operator code, to make sure the SparseMPO and MPOHamiltonian structs are defined before the braille function. If someone comes up with a better solution, please share.

[codecov]

Codecov Report

Attention: Patch coverage is 46.15385% with 28 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/operators/show.jl	46.15%	28 Missing ⚠️

Files with missing lines	Coverage Δ
src/MPSKit.jl	100.00% <ø> (ø)
src/operators/abstractmpo.jl	85.92% <ø> (+23.89%)	⬆️
src/operators/show.jl	46.15% <46.15%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[lkdvos]

Definitely makes sense to have a separate file, this is a good solution. Maybe it might be reasonable to simply define a operators/show.jl file and move all of this there?

I'm also okay with leaving it as it is now.

[VictorVanthilt]

I moved the abstract mpo show methods, together with braille method to operators/show.jl. I also added a docstring to the braille function to make it less of a secret to the users.

[lkdvos]

Good for me if tests pass