Create a dyad with properties "stickness", "ballness" and "orientation".
Cast a length 3 "vector" to a Dyad2D.
Cast a length 3 "vector" to a Dyad2D. This should only be called if the vector was originally created by downcasting of a Dyad2D, because the internal representation of Dyad2D is not part of the interface.
Create a zero-filled Dyad2D.
Create a ball tensor of size "diameter".
Create a stick tensor of stickness "length" and "orientation".
Factory for creating Dyad2Ds.