refactor sto

[wpbonelli]

as per #206. also clean up the maxbound recalculation and update the pixi env

[wpbonelli]

unnecessary now. fixed in the tentative v2 dfns, which codegen can soon consume

...
  period:
    fields:
      storage:
        type: string 
        longname: storage state
        description: keyword to indicate that stress period iper is steady-state. steady-state conditions
          will apply until the transient keyword is specified in a subsequent begin period block. if the
          csub package is included in the gwf model, only the first and last stress period can be steady-state.
        optional: true
        valid:
          - steady-state
          - transient