Rename Exponential Map in Code to generalize notion of retractions

If one wants to use a different update instead the exponential map, e.g. for unit vector just re-normalization.

In the Trustregion algorithm the "exp" is directly used where a more general retraction could be nicer https://gitlab.mn.tu-dresden.de/osander/dune-gfe/-/blob/master/dune/gfe/mixedriemanniantrsolver.cc#L485

          for (size_t j=0; j<newIterate[_0].size(); j++)
              newIterate[_0][j] = TargetSpace0::exp(newIterate[_0][j], corr[_0][j]);

          for (size_t j=0; j<newIterate[_1].size(); j++)
              newIterate[_1][j] = TargetSpace1::exp(newIterate[_1][j], corr[_1][j]);

Solution:

• rename general usage from exp to retr
• Manifolds should implement retr and point to the corresponding algo

The retraction pair exp and log should be also provided for other retraction, therefore log should be also replaced.