* disjoint_subsets: make clear and sizde functions noexcept

* disjoint_subsets: add get_number_of_sets function, documentations and tests

* disjoint_subsets: add get_size_of_set function, documentation and tests

* Split disjoint_subsets in a hierarchy.

Modify disjoint_subsets to make it a valid base class.
Add new class disjoint_subsets_sized, with information about number of
subsets, and size of each subset.
Add necessary unit test for the new class.

* Replace tabs by spaces.

* Remove virtuals from disjoint_subsets, and modify
disjoint_subsets_sized.

Now disjoint_subsets_sized is implemented in terms of disjoint_subsets,
instead of inherit from it.