Cleanup plan - move some things to borsar, remove some function that are never used

[mmagnuski]

Remove entirely

• sarna.cluster.get_neighbours() - mne has many FT neighbours now, but these might have been manually curated by me some time ago, so I should check before removing
• sarna.utils.extend_slice if not used elsewhere (here or DiamSar)
• sarna.viz.plot_topomap_raw seems not to be used
• sarna.utils.find_files - could be useful when improved, but currently not used
• sarna.events.create_middle_events
• sarna.cluster.plot_neighbours() becasue mne already has mne.viz.plot_ch_adjacency
• sarna.viz.plot_cluster_heatmap - this functionality is offered now by borsar.Clusters
• remove viz.heatmap() and its dependencies - its already in borsar

Relatively easy to move to borsar

• proj.find_dropbox and get_valid_path
• viz.highlight

Easy finish

• sarna.colors might be useful in future, but currently does not offer anything worth using

Cluster-level stuff will require more work to clen up and unify:

• permutation_cluster_t_test() later maybe just permutation_cluster_test
• _permutation_cluster_test_3d + change its name (it is now more general than 3d)

Check and plan

• stuff in sarna.gui - is any of that works still?
• events.apply_artifacts_to_tfr, events.read_set_events()
• I have cleaned sarna.utils.mne_types - it is used in two places in sarna but I'll have to see if it will be actually useful