Grid array input packages (READARRAYGRID) not supported

[mjreno]

Grid input (e.g. CHDG) packages are currently not supported:

• test(chdg): support writing grid array input package #237
• Current failure is in structure.py structure_array(), which assumes list style formatted input, e.g. https://github.com/modflowpy/pyphoenix-project/actions/runs/18855133820/job/53801055200?pr=237
• Current test is formatting grid based input as:

head = {0: np.full((nlay, nrow, ncol), DNODATA, dtype=float)}
head[0][0, 0, 0] = 1.0
head[0][0, 9, 9] = 0.0
chd = Chdg(parent=gwf, head=head)

Support for READASARRAYS based packages will be evaluated in separate PR / issue.