Refactor diagnostics module

[JulioAPeraza]

Summary

Following the changes in #765, where we adopted get_clusters_table from nilearn, we plan to get the label maps from that function as well. That output should be available in the next nilearn release.

Additional details

This will potentially reduce the computation time and the number of lines of code, as ndimage.label won't be re-run.

Next steps

For nilearn>0.10.0, add return_label_maps parameter:

clusters_table, labeled_cluster_arr = reporting.get_clusters_table(
    target_img, 
    stat_threshold, 
    return_label_maps=True,
)