Un-duplicate admin management code on Inflow adapters

See the below comment by @p-offtermatt in the PR #413 (Add admin management on hydro, ibc, skip and mars adapters):

> Seems weird to duplicate the code in each adapter when it's the same function, worth rolling into a utility we can just import? seems also safer to avoid making a small mistake in one of the management functions

Creating this issue to keep track that we should - as possible - not duplicate the admin management code in each Inflow adapter.

Start by reviewing PR #413 and locating the admin management implementations in the hydro, ibc, skip, and mars Inflow adapters. Compare their shared behavior and determine where a reusable utility belongs; done means the duplicated management code is consolidated where practical and all affected adapters retain their existing behavior.

Written by the indexing model from the issue text.