Improve implementation of segmentation algorithms (and include alternatives)

[gilbertocamara]

The current version of sits_slic() is slow and fails to work in some circumstances. The function has its own memory management which conflicts with that of the main sits algorithm. It would be good to improve it with a version that would be consistent with the way sitsdoes classification, using blocks and parallel processing.

[M3nin0]

Dear @gilbertocamara,

Thanks a lot!

As part of this review, we should consider review the DTW implementation (#1296) as we discussed.