Modified implementation in favor of a constant eccentricity manifold

Modified implementation in favor of a constant eccentricity manifold
Unit tests, comments, and pictures were modified accordingly.

Minor modifications

 - Adjusted style.
 - Used `std_cxx14` instead of `std` for `make_unique()`.
 - Added guard in `pull_back()` to prevent NaNs in `std::acos()`. This
 may happen due to numerical errors: when the argument of acos() falls
 outside the range [-1,1].

Included a special case for pull_back()

If the first coordinate is exactly 0, the center of the manifold is
returned.