Bugfix: yearsets frequency curve calculations fail

[ChrisFairless]

Changes proposed in this PR:

• Small bugfix to yearsets. An Impact.at_event object needs to be a numpy array, or it can't be reindexed in the Impact.calc_freq_curve calculation. The yearsets class creates these as lists so frequency curve calcuations currently fail. From the documentation they are clearly supposed to be arrays when they are calculated in the compute_imp_per_year method. The fix is one line.

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[chahank]

Great! Can you please just adjust the docstring to return a np.ndarray?

[ChrisFairless]

Done 👍