Add method for getting HUC-12s
Add method for getting HUC-12s

Given a HUC code, finds all HUC-12s contained within. For a HUC-8
like Necanicum, will find all HUC-12s starting with '17100201%'.
Must be given HUC-8, HUC-10, or HUC-12 code. In case of HUC-12,
will match exactly one HUC-12. The response is always in a list,
for easy reuse with existing code.

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