Birth Period in icarInventoryClassificationType doesn't validate as per description

[cookeac]

The birthPeriod in icarInventoryClassificationType was intended to represent an approximate birth period for a number of animals with an ISO8601 interval (effectively a range between two dates, but there are extensions that allow year or year and month).
I had originally drafted this as an icarDateType which is just string format date. However, most validators only accept RFC-3339 dates in this context, and not intervals.

I propose to make this a string, with a pattern that reflects the description.

[cookeac]

Resolved by #334 (hopefully)