Mask context

[ceesem]

This version adds a number of changes

• Resampling skeletons (thanks to Emily!)
• Robust convenience functions for SWC export from meshworks using arrays or annotation names.
• SWC export has resampling as an option
• SWC export can now take BytesIO/StringIO objects for CloudFiles compatibility
• Function to aggregate a per-mesh vertex property to a per-skeleton vertex property.
• A Meshwork object now has a mask context, so that you can do patterns like:

  with nrn.mask_context( my_mask ) as nrn_m:
  data = measure_stuff( nrn_m )

  without permanently changing the object.

• Various bug fixes where indices were not handled correctly, for example going from a MeshIndex array to SkeletonMask array.