Optional operations for DisjointSetsForest

[nbro]

• size (number of elements in the data structure)
• contains (checks if an element is in the data structure)
• sets (number of sets in the data structure: may be useful for example to understand how far are we to have just one set, etc)
• Erase or clear the data structure
• Deletion of an element from a set (and from the data structure)
• Deletion of a whole (disjoint) set
• Pretty-print a set (similar to how we're pretty-printing a BST)
• Counting elements in a set (this could also be a property)