STO (storage) period specification

[mjreno]

This is an issue to track the evaluation of alternative ways of specifying dynamic (period) input for the storage package. The intent is to more closely align storage with existing patterns of specifying dynamic input, if possible, or to formalize a new general approach that both storage and other packages can use.

Currently storage period input comes in the form of two optional keywords, steady-state or transient. This specification is more like you'd expect to see in an OPTIONS block- does this represent a more generic class of dynamic input for utilities and packages?

An list style specification is being evaluated here:

• refactor(sto): convert storage dfn to use recarray for period input MODFLOW-ORG/modflow6#2572