Todo's for the level-set package:

* Allow for non-uniform grid in fast marching by optionally specifying a
  vector of grid spacings for each space dimension.

* Boundary length / curvature / shape derivative?

* Fix mesh component numbering and add outer boundary?

* Split off so_ routines into a general descent-based optimisation framework?
  The relation to level-set and shapes is only in very few places.
