Tests for model comparison diagnostics + ece as a diagnostic metric

[Kucharssim]

Adds tests for model comparison diagnostics (mentioned in #335 (comment))

Also moved expected_calibration_error() into diagnostics.metrics and made the interface a little bit more consistent with the new style.

[codecov-commenter]

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Files with missing lines	Coverage Δ
bayesflow/diagnostics/metrics/__init__.py	100.00% <100.00%> (ø)
.../diagnostics/metrics/expected_calibration_error.py	100.00% <100.00%> (ø)
bayesflow/diagnostics/plots/mc_calibration.py	100.00% <100.00%> (+77.27%)	⬆️
bayesflow/utils/__init__.py	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

[paul-buerkner]

Looks great! Thank you!