Add derivative and curvature to PathSeg

[Sh4rK]

This is meant to start thinking, rather than as a finished PR.

The goal is to allow PathSegs to be differentiated, as it can be useful in some cases to treat all kinds of paths uniformly.

This PR implements two possible solutions, but there are more I can think of:

1. Derivative of a line is a degenerate line (in this PR)
   Pros:

   • easy to implement without many changes

   Cons:

   • is basically a hack

2. Add ConstantPoint to PathSeg
   Pros:

   • PathSeg will be closed regarding to differentiation
   • smallish change

   Cons:

   • ConstantPoint doesn't really make sense in PathSeg

3. Add a deriv_at method (better name welcome) to PathSeg (in this PR)
   Pros:

   • small change

   Cons:

   • limited to the evaluation of the first derivative
   • odd to have it only on PathSeg

4. Add a new trait that allows evaluating the first (second?, third?) derivative
   Pros:

   • more general solution than above

   Cons:

   • limited to the evaluation of the first (second?, third?) derivative

Any thoughts, more ideas? Is something like this even welcome? :)