Add optional max, min number of children to each item in the children strand

[thclark]

Specifically allow for specifying a max_num (default -1, or inf) or a min_num (default 0) to each item in the children strand of a twine.

NB use max_num rather than minimum because it's unambiguous as to what the max refers to.

• Update the twine schema
• Update the children validation method to check against quantity limits
• Add unit tests covering scenarios of too many, too few, correct number with entries, correct number without entries

[cortadocodes]

What's the use case of this @thclark?