Lp dist field

This is an implementation of the Lp-dist field approach for smoothly approximating the distance to a given surface.

Reference

@article{belyaev2013signed,
  title={Signed Lp-distance fields},
  author={Belyaev, Alexander and Fayolle, Pierre-Alain and Pasko, Alexander},
  journal={Computer-Aided Design},
  volume={45},
  number={2},
  pages={523--528},
  year={2013},
  publisher={Elsevier}
}