winding order, to account for the possibility that different vertex sets
require different orders.

change flag/create a copy themselves

compressed voxelwise EVs

- (..., 3, 3) full diffusion tensors
- (..., 6) unique components
- tuple of eigenvectors & eigenvalues

There should be full test coverage of these new functions,
but coverage.py is currently not working for me, so not 100% sure.

default to returning a flat list of matches. subtree-related fixes to scan
function

operators to Match class so it can be ordered. Protect Match attributes via
@property

mutate canonical version

for which an optional variable is not present

actually exist on disk (and corrsepond to files).