Add identification functions

[JulStraus]

New exported functions has_opscenarios and has_repr_periods check whether a TimeStructure or TimeStructurePeriod contains OperationalScenarios or RepresentativePeriods. They work on all structures, including TwoLevelTree and the single-period wrappers.

This PR hence closes #78.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/TimeStruct.jl	100.00% <ø> (ø)
src/structures.jl	80.00% <ø> (ø)
src/utils.jl	86.92% <100.00%> (+2.55%)	⬆️

🚀 New features to boost your workflow:

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

[trulsf]

Looks fine. It could be a possibility to have a version for a generic TimeStructure that defaults to false, saving some lines, but I think I like to have the explicit false declarations.

[JulStraus]

Looks fine. It could be a possibility to have a version for a generic TimeStructure that defaults to false, saving some lines, but I think I like to have the explicit false declarations.

I considered it, but thought there is no harm in being explicit. A default false may be a problem in a future instance if we implement new time structures. In this situation, I think it is better to throw a MethodError.