SRSF / Clustering based on phase variability

[astamm]

K-mean alignment using the SRSF framework is currently done via fdasrvf::kmeans_align() which does not offer the possibility to perform clustering based on phase variability.
The output distances to center are based on distances on warping functions in order for fdadist() to just work with the SRSF framework and phase distances but we need need to better handle this.