change to UnitQuaternion.interp()

[petercorke]

On GH version, not yet PyPI version.

Old behaviour

qf.interp(s)  # s=0 gives 1<0>, s=1 gives qf
q0.interp(s, dest=qf) # s=0 gives q0, s=1 gives qf

was not self consistent, and differed from the SO3/SE3 semantics.

New behaviour

qf.interp(s)  # s=0 gives 1<0>, s=1 gives qf
qf.interp(s, start=q0) # s=0 gives q0, s=1 gives qf