Rehome `_numpy_allclose` so it can be decorated

I would like to decorate the function _numpy_allclose() with _manage_log_level_via_verbosity as a standard & consistent way to manage the output of the log messages there with overriding by the verbose kwarg.

But as-is this cannot be done because _numpy_isclose() lives in the functions module & importing the decorator would lead to a circular dependency on imports, as per the comment I added there:
https://github.com/NCAS-CMS/cf-python/blob/7fed0abf184093024eb35d17036a08dc812c8f47/cf/functions.py#L1067-L1070

We tentatively agreed we should move the function to another module. It is internal so the location is not user-facing but we should try to collect it into the most sensible module as a new home.

Inizia in functions.py, intorno al commento collegato alle righe 1067–1070, e segui _numpy_allclose(), _numpy_isclose() e _manage_log_level_via_verbosity. Individua un modulo che eviti l’importazione circolare, sposta lì la funzione interna e verifica che il decoratore possa essere applicato senza compromettere gli import o il comportamento esistente.