ShortfallEvents and LOLEv Implementation

[akrivi]

This PR adds a new ShortfallEvents result specification that records shortfall events directly during simulation.

We define an "adequacy event" as a set of event-periods that are contiguous at the highest available temporal resolution [1].
Events are recorded separately for each Monte Carlo sample at both system and regional levels.

ShortfallResult does not retain sufficient information to reconstruct individual events.
 At the same time, obtaining this information from ShortfallSamplesResult requires storing and postprocessing the full regions × timestamps × samples shortfall tensor.


ShortfallEvents instead stores event boundaries and accumulated unserved energy, avoiding that dense tensor, which provides a more compact representation for event analysis, particularly when shortfalls are sparse.

The implementation adds the following metrics at both system and regional levels:

1. LOLEv: the expected number of shortfall events per simulation horizon, estimated by averaging event counts across Monte Carlo samples [2].
2. MeanEventDuration: mean duration across all recorded events.
3. MaxEventDuration: maximum duration among all recorded events.
4. MeanEventEnergy: mean unserved energy across all recorded events.
5. MaxEventEnergy: maximum unserved energy among all recorded events.

With the implementation of LOLEv, all metrics defined in [2] are now included in PRAS.

Additionally, this PR extends PRASFiles to support JSON export of ShortfallEventsResult. This includes both aggregate event-level metrics and optionally raw event records at the system and regional levels.

[1] Stephen et al. (2022), "Clarifying the Interpretation and Use of the LOLE Resource Adequacy Metric"
[2] NERC (2018), "Probabilistic Adequacy and Measures Technical Reference Report"