[major_surface_waters] sea_stage dispatch uses isinstance(..., float) and so mishandles an int stage

[bdestombe]

major_surface_waters.py:102:

if isinstance(sea_stage, float):
    ...  # constant-head branch
else:
    ...  # time-series branch, head="sea_stage", timeseries=sea_stage

An integer stage — chd_ghb_from_major_surface_waters(ds, gwf, sea_stage=0) — is not a float, so it falls into the time-series branch and is passed to chd.ts.initialize(timeseries=0), which is not a valid (time, stage) series. The caller gets an obscure flopy error instead of a constant 0 m head.

isinstance(sea_stage, (int, float)) or np.isscalar(sea_stage) would dispatch on what the branch actually distinguishes, namely scalar-versus-series.

Low impact today (the 09pwnmodel2 call site always passes a list), but it is a trap for any new caller. Found while writing the nhflotools test suite; deliberately not pinned by a test.