Constrained clustering of SOM map

[gilbertocamara]

Describe the new API function requested
When building a SOM map with generic classes, it is useful to break this map into sub-classes by making a re-labelling operation where a constrained clustering technique (such as SKATER) is used to find new clusters in the map.

Associated sits API function
sits_som_reclass()

Additional context
Add any other context about the new function here.