Fix single parameter plots

[han-ol]

Fixes #297
It is sufficient to make sure plot_utils.make_figure() always returns an ndarray of axes.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
bayesflow/diagnostics/plots/loss.py	0.00%	1 Missing ⚠️
bayesflow/diagnostics/plots/recovery.py	0.00%	1 Missing ⚠️
bayesflow/utils/plot_utils.py	0.00%	1 Missing ⚠️

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Files with missing lines	Coverage Δ
bayesflow/diagnostics/plots/loss.py	22.22% <0.00%> (ø)
bayesflow/diagnostics/plots/recovery.py	22.22% <0.00%> (ø)
bayesflow/utils/plot_utils.py	20.77% <0.00%> (ø)

... and 24 files with indirect coverage changes