Un-duplicate admin management code on Inflow adapters

[arlai-mk]

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.