[ENH, MRG] Add volume/tumor segmentation

[alexrockhill]

Within-MNE solution for mne-tools/mne-python#10860.

Here's how it works:

find_tumor.webm

[alexrockhill]

TO DO:

• Write tests
• Add mark all/globally for vasculature
• Add brainmask also for vasculature
• Add example data (same as for grids potentially
• Write example

[alexrockhill]

The plan is to add a tumor and microgrids dataset in a couple weeks/months and make an example that combines this PR and #10 to show how to locate contacts from a surgical image over a tumor.

[alexrockhill]

Thoughts on merging this now and then adding it in an example when #10 merges?

We already closed the issue on segmenting a tumor so it might make sense to merge.

[larsoner]

Seems reasonable to me