Feature: Restricting the number of subsections of a specific 'type'

With basic cardinality rules implemented for `Property -> Values`, `Section -> Properties`, and `Section -> Sections`, the topic came up if we would also like to have more specific rules, especially for `Section > Sections`. One idea that came up is restricting the number of subsections of a specific `type`.

_Originally posted by @achilleas-k in https://github.com/G-Node/python-odml/issues/361#issuecomment-616523135_