Extraction of the operational structures of a time structure

[JulStraus]

Problem statement

As outlined in #79, we have differing field names for the operational time structures within a time structure (Vector{OP}). It would be in general beneficial to provide a function for extracting the operational time structure within a TimeStructure. This function could be used both for TimeStructure{T} and TimeStructurePeriod{T}, as used for, e.g., for extracting the field operational of a TwoLevel or the field operational of a StrategicPeriod or StratReprPeriod. We have an internal function _oper_struct which is used for TimeStructOuterIter{T}, but we do not extend it for the other created types.

This function could also be beneficial to be exposed to the user if they want to implement specific constraints that are only present for a given time structure as the level constraints in EMB.

Potential solution

Create an equivalent of _oper_struct for TimeStructure which is exported.