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.

Start in functions.py around the linked comment at lines 1067–1070 and trace _numpy_allclose(), _numpy_isclose(), and _manage_log_level_via_verbosity. Identify a module that avoids the circular import, move the internal function there, and verify that the decorator can be applied without breaking imports or existing behavior.