Use `fmm` to find perimeter of throats

[jgostick]

We currently use the marching_cubes method to get more accurate surface area values. It got me thinking that throat perimeter could/should be done more accurately too.

• Find the 3D image containing the two neighboring regions.
• Identify the throat voxels shared by the two regions (this is already done in getnet)
• Find the perimeter voxels with dt < 2
• Perform fast marching on this from a random starting point
• The perimeter will be 2x the maximum value since the march goes in both direction and will meet in the middle on the other side

[jgostick]

This is complicated and I think it can be a feature that we add to a future release, so I'm removing from this milestone