Implement `metric` parameter for cross_validation_across_time

[skjerns]

it would be nice to have a parameter with which we can use a different scoring algorithm for the function.

The parameter should accept either a string keyword or alternatively a function. if it's a string, it should take a function from https://scikit-learn.org/stable/modules/model_evaluation.html with the corresponding name

for the top-N you can use https://scikit-learn.org/stable/modules/generated/sklearn.metrics.top_k_accuracy_score.html

so you can call cross_validation_across_time(..., metric='top-3')

the default should be metric='accuracy'

Best,
Simon