Add possibility to add callable or other metrics within the class.

[quentinhaenn]

This enhancement task is supposed to add the following enhancements:

• Add the possibility to switch between metrics within the code (e.g. use Manhattan instead of Euclidean metric)
• Add the possibility to use a user-defined metric (e.g. calling a user-defined my_dissimilarity function on input data)
• Refactor the clustering method and put clustering_exact and clustering_approx methods in a separate file, to prepare for Curgraph adding (v2.0.0)

[quentinhaenn]

UPDATE :

We also added the possibility to handle custom MDS solvers within the package.