Submodule with various operations on some common splines.
More or less functional, though the comments and docs could stand some improvement.
A few basic splines are unimplemented (TCB, uniform B splines), but not really high-priority.
It could stand a helper module, I imagine, since putting all the pieces together (say, integrands and arc lengths) is rather non-obvious.
Tested on Lua 5.1.
The arc_length module depends on some integrators
support, which is in the rather unstable number_ops directory (logically it's four or five submodules, I think). It is very likely this will
move, say to a "solvers" submodule, but I'm still thinking about it.