sphere.py: remove argument `t` from `exp` and `ret`
sphere.py: remove argument `t` from `exp` and `ret`

The scaling factor `t` of the tangent vector `U` can be premultiplied by
the user.

Signed-off-by: Niklas Koep <342d5290239d9c5264c8f98185afedb99596601a@gmail.com>

pymanopt/pymanopt,pymanopt/pymanopt,nkoep/pymanopt,j-towns/pymanopt,nkoep/pymanopt,tingelst/pymanopt,nkoep/pymanopt