Restructuring of discounting

[JulStraus]

PR #71 introduced a new discounting method. Within the PR we identified potential for improving the overall code structure. This is now done within this PR. It can still be improved, e.g., providing types for the different discounting methods through different Discounter types, if desired.

I furthermore did a change to API structure which is in practice rather miniscule.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.96970% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/discount.jl	96.96%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/discount.jl	98.36% <96.96%> (+7.13%)	⬆️

... and 10 files with indirect coverage changes

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[JulStraus]

I updated the identification of strategic periods with a new method which also throws errors if a wrong value is specified. One issue in this case is that with the current method, we cannot say with a 100 % certainty that the TimePeriod is actually within the TimeStructure as we only compare the sp value, i.e., comparing Ints.

We can, if desired, extend the overall approach as well as the identification of a strategic period is in practice potentially something of general relevance. One issue at the moment is that is rather inefficient when discount or objective_value are used on OperationalPeriod level or comparable.

[JulStraus]

I identified one approach for a proper identification:

filter_sp(sp::AbstractStrategicPeriod, t::TimePeriod) = t in sp
filter_sp(sp::AbstractStrategicPeriod, t::AbstractOperationalScenario) = t in opscenarios(sp)
filter_sp(sp::AbstractStrategicPeriod, t::AbstractRepresentativePeriod) = t in repr_periods(sp)

function _sp_period(t, ts::TimeStructure)
    sps = collect(strat_periods(ts))
    filter_fun(sp) = filter_sp(sp, t)
    per = findfirst(filter_fun, sps)
    isnothing(per) && throw(ErrorException("Time period not part of any strategic period"))
    return sps[per]
end

This approach identifies whether the node is exactly in the time structure compared to just checking that the sp (and branch) are correct. Note that the system also works with TwoLevelTree.

[trulsf]

I like the simplifications and doc updates. Currently, I can live with the version that identifies strategic period by index.