Make SparseMatrix implement Iterable.
Iterating on empty SparseMatrix unit test.
Isochrone: Add SPTWalker to visit SPT geometry.
Isochrone: Make use of the SPTWalker in rendering.
Correct wrong comment.
Isochrone: use generated origin edges in SPT walk.
Isochrone: Use generics for Z instead of double[].
Vectorizer: extract grid sample mgt to own class.
Vectorizer: delegate coordinate mgt to sample grid.
Vectorizer: Remove generics for user data.
Isochrone: Extract grid sampling of isoline build.

Split the Z metric in two (unrelated) parts:
- The proper Z metric (z0 plane intersection detection, z interpolation method)
- The accumulative sampling metric/behavior (decay function, closing process).