EAMxx: add capability to output time-invariant quantities (possibly after 1 time step)

It could be useful for quantities we know don't change, say PHIS or some constants.

Maybe a generalized feature here is to enable outputing a single snapshot (not recurring) of some variables, say after X steps.

No file or test is named. Start by locating the EAMxx output configuration and existing time-stepped output handling, then clarify whether the intended behavior is a one-time snapshot after X steps, support for time-invariant quantities, or both. Done should be demonstrated by a defined configuration and validation that the selected quantities are emitted once at the requested time.

Written by the indexing model from the issue text.