Fix get_all_periods for RepresentativePeriods and OperationalScenarios

[JulStraus]

Problem

get_all_periods! called itself on the raw inner time structures (ts.operational, ts.rep_periods, ts.scenarios). This meant:

• Nested periods lost the strategic, representative or scenario indices of the levels above them.
• Operational periods were only collected for TwoLevel. For a top-level RepresentativePeriods or OperationalScenarios, they were missing from the period mapping.

Solution

The function now takes two arguments: the current period and the time structure it belongs to.

It walks down through the period objects (strategic_periods, repr_periods, opscenarios), and the time structure argument decides which method runs. At the SimpleTimes level it collects the operational periods, so each one keeps its full index chain. New tests in test/test_utils.jl cover all combinations of TwoLevel, RepresentativePeriods, OperationalScenarios and SimpleTimes.

[JulStraus]

One think I forgot to mention in the description above: I did not increase the version number, as I have a pretty much finished PR for #55 which will be registered together with this PR.