Rename internal use functions

(originally posted on this comment)

I think we need to distinguish among exported method, non-exported method and internal use method.

• exported method
  • e.g.RotXYZ, nearest_rotation
  • These methods should be maintained and documented.
• non-exported method
  • e.g. Rotations.params, Rotations.jacobian
  • These methods also should be maintained and documented.
  • The reason for non-exported is just avoiding name conflict.
• internal use method
  • e.g. _pure_quaternion
  • These functions are regarded as an internal method, and may be changed without breaking release.
  • In Rotations.jl, many functions should be renamed with underscore _. (e.g. vee -> _vee, lmult -> _lmult)
    • I'm not sure if such renamings would be a breaking change.
    • See blue guide for more information.