add twisted orb geometry

[Kwasniok]

Similar to swirl geometry but with exponential falloff. See twisted_orb_geometry.nb (requires Mathematica)

• [x] add skeleton class
• [x] add metric
• [x] add jacobians
• [x] add ray passing through
• [x] add Christoffel symbols1 & 2
• [x] add unittest
• [x] add documentation