Feat/metric crps

[dennisbader]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Note: this is a continuation of @Boubker10 's work on #3088.

Fixes #1112.

Summary

Added two new metrics for probabilistic time series forecasting:

• crps (Continuous Ranked Probability Score): a per-time-step proper scoring rule that measures the compatibility of a predictive sample distribution with a scalar observation. Reduces to the Absolute Error (AE) when all samples are identical.
• mcrps (Mean CRPS): mean of crps over the time axis, analogous to MAE for probabilistic forecasts.

Both metrics follow the existing metric API (support for component_reduction, series_reduction, time_reduction, multi-ts, multivariate).

Other Information

Formula used (energy score formulation):

CRPS = (1/N) * Σ|x_i - y| - (1/(2N²)) * Σ_i Σ_j |x_i - x_j|

Tests added in darts/tests/metrics/test_metrics.py covering:

• raises ValueError for deterministic input
• perfect prediction → CRPS = 0
• numerical correctness against manual calculation
• multivariate and multi-ts consistency

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.24%. Comparing base (aa16909) to head (ddd3206).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3089      +/-   ##
==========================================
- Coverage   96.30%   96.24%   -0.07%     
==========================================
  Files         160      160              
  Lines       17263    17280      +17     
==========================================
+ Hits        16625    16631       +6     
- Misses        638      649      +11     

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