diagnostics: log inputs, computed max_order, and warn when write_new_parameters produces no output

[Sebokolodi]

Motivation

write_new_parameters currently derives the number of output files from a max_order heuristic on new_coeffs:

max_order = (
    np.sum(np.any((new_coeffs != 0.0) & (~np.isnan(new_coeffs)), axis=(1, 2))) - 1
)
for i in range(max_order + 1):
    pf.writeto(...)

When new_coeffs is entirely 0.0 or NaN across every plane, max_order resolves to -1, the write loop becomes range(0), and no files are written, no exception is raised, no log line is emitted.

Downstream consumers that assume newcoeff*.fits exist can be hurt badly by this silent path. In the CIRADA POSSUM pipeline (POSSUM_Polarimetry_Pipeline/pipeline/modules/fracpol3d_Irescale.py), a subsequent delete-then-rename block would destroy the input coeff*.fits files and only crash much later with a FileNotFoundError at pf.getdata(coeff0.fits), far from the root cause. That has now been fixed defensively on the pipeline side (POSSUM_Polarimetry_Pipeline#33), but the underlying silent no-op inside RMtools deserves visibility here too.

Observed: tile 6614, 943MHz, RMtools 1.4.11 in production — the writer produced zero output files with no warning; only became visible when a downstream getdata crashed.

Changes (all diagnostics — no behaviour change on the happy path)

• Add a module-level logger and a small _summarize_coeffs helper that returns a compact one-line descriptor of a coefficient array (shape, dtype, total elements, nan count, zero count, finite-nonzero count).
• rescale_I_model_3D: log input coeffs summary at the start and output new_coeffs summary at the end (INFO).
• write_new_parameters: log input summary and the computed max_order up front; log every file written with its full path; log a final total-files-written count at the end (INFO).
• write_new_parameters: when max_order < 0, emit both logger.warning and warnings.warn(UserWarning) with a clear message identifying the condition ("every plane of new_coeffs is 0.0 or NaN"), then return explicitly. The historical behaviour of writing no files is preserved; the reason is now visible even to callers that haven't configured logging.

Non-goals

• No behaviour change on inputs that previously wrote files. Same files, same content, same order.
• Not changing the max_order heuristic itself. If it turns out the heuristic is wrong (e.g. failing on valid low-order fits under some conditions), that's a separate fix.
• Not raising instead of warning. warnings.warn(UserWarning) is the loudest visibility change that doesn't break existing callers that treat the empty-output case as valid.

Test plan

• Existing tests should pass unchanged (only additive log/warning code paths).
• Manually construct new_coeffs filled with NaN, call write_new_parameters — expect a UserWarning and a logger warning, no files written (as before).
• Manually construct valid new_coeffs — expect INFO log lines showing the summary, max_order, and per-file paths, plus the normal FITS output files.