API for "Surface curvature lines"

Слиты nikitronn запросил слияние surf_curvature_lines в master

Created by: portnov

As it is that questionable, I decided to move it to Sverchok-Extra. As a side feature, it allowed to use ODE solvers from scipy.

This PR contains only some API updates required for the node in Sverchok-Extra.

This node draws principal curvature lines (https://en.wikipedia.org/wiki/Principal_curvature#Line_of_curvature) on a surface.

I'm not yet sure how usable this node is, because there are some problems:

  • It is relatively slow, especially if you want high precision
  • It outputs just series of points, not Curve objects (though one can use Polyline or Cubic line nodes to make interpolated curves)
  • Lines of curvature may sometimes take a 90 degree turn - this is due to differential geometry fundamentals: it does not differentiate "first" and "second" curvature directions, so at umbilical points they are indiffirentiable at all
  • Lines of curvature may unexpectedly be directed to the opposite direction - again, because differential geometry does not differentiate between a direction and it's opposite.

Let's this be in a PR for a while, see for opinions and test results...

Screenshot_20200517_120358

Preflight checklist

Put an x letter in each brackets when you're done this item:

  • Code changes complete.
  • Code documentation complete.
  • Documentation for users complete (or not required, if user never sees these changes).
  • Manual testing done.
  • Unit-tests implemented.
  • Ready for merge.