Refactor/clean module of discrete curves

[ninamiolane]

There are a few tasks that remain to be done to clean-up or refactor the module of discrete curves:

• Address the comments from @Florent-Michel 's PR General elastic metric #1078 to clean the code:
  • In particular, we need to have consistency between SRVMetric and ElasticMetric(1, 0.5), which is not the case at the moment.
• Address the problem that "A. Srivastava, E. Klassen, S. H. Joshi and I. H. Jermyn, in "Shape Analysis of Elastic Curves in Euclidean Spaces," quotient the space of curves by the action of the translation, but our implementation does not (while we are citing their paper).
  • We could made the quotient by the translation as an option.
• Use the structure of quotient metric to organize the code.

[ninamiolane]

The class QuotientSRVMetric should not inherit from SRVMetric.

Indeed, with this inheritance, whenever a method is not found in QuotientSRVMetric, it is taken from its parent class SRVMetric - which is not correct.

For example, QuotientSRVMetric does not have a log implemented, and should not take the log from SRVMetric.

[ninamiolane]

Same for ClosedSRVMetric. Using QuotientSRVMetric or ClosedSRVMetric within FrechetMean will run, but the estimated mean will not be the frechetmean corresponding to these metrics --- because of this inheritance.

[ninamiolane]

@alebrigant what do you think about this paper to compute the distance (only the distance) in the quotient SRV space:
https://arxiv.org/pdf/1501.00577.pdf: PRECISE MATCHING OF PL CURVES IN RN IN THE SQUAREROOT VELOCITY FRAMEWORK (Sahini, Robinson, Klassen - 2015)

[ninamiolane]

ElasticMetric does not have log, exp and geodesics, which should be added. Florent-Michel should have code for the geodesic.

[ninamiolane]

'DiscreteCurves' object has no attribute 'random_uniform'

[ninamiolane]

DiscreteCurves should also inherit from Landmarks and have a fixed number of sampling points. This would be more consistent with the code base, and avoid math.inf in the dimensions of the manifold.

[ninamiolane]

• SRVMetric should inherit from ElasticCurve
• srv_transform and srv_transform inverse should overwrite f_transform and f_transform_inverse

[ninamiolane]

• f_transform_inverse should be unit-tested