Review DTW implementation

**Describe the requested improvement**
The current implementation of DTW is slow. We should revise it to include a better version (e.g., [UCR Suite](https://www.cs.ucr.edu/~eamonn/UCRsuite.html)). 

**Associated sits API function**
`sits_som_map` and `sits_segmentation`

**Additional context**
This issue is related to the #1292 
