class_docstr: Class used to plot points on the manifold SPD(2).
  
  Elements S of the manifold of 2D Symmetric Positive Definite matrices
  can be conveniently represented by ellipses.
  
  We write :math:`S = O D O^T` with :math:`O` an orthogonal matrix (rotation)
  and :math:`D` a diagonal matrix. The positive eigenvalues, i.e. the elements of
  :math:`D`, are the inverse of the length of the major and minor axes of
  the ellipse. The rotation matrix :math:`O` determines the orientation of the
  2D ellipse in the 2D plane.
  
  Parameters
  ----------
  k_sampling_points : int
      Number of points to sample on the discretized ellipses.
code: def __init__(self, k_sampling_points=100):
      self.k_sampling_points = k_sampling_points
  
imports: import matplotlib.pyplot
  from mpl_toolkits.mplot3d import Axes3D
  import geomstats.backend
label_desc: Use image node_id 1 to create a new Ellipses object  with example: obj = Ellipses(k_sampling_points)