Closes #37. Sister PR to NCAS-CMS/cfdm#34, with largely the same purpose & implementation, & using much of the corresponding functionality there (by import where possible) but with the following key differences:

• cf-python applies info & _debug keywords as well as just verbose to enable filtering on some level of verbosity. These keywords have all been converted to verbose for consistency, & appropriate level have been given to any call being filtered as such, e.g:

  if _debug:
      print("my message")

  has been changed to the following, so it only emerges with debug logging configured locally via verbose keyword (see next point), or if not globally:

  logger.debug("my message")

  where info levels have been mapped appropriately, as discussed, namely:

  Min. verbosity	(Old) info level	New verbose level
  warning	0	1
  info	1	2
  detail	2	3
  debug	3	-1

• unlike in cfdm there are cases where the verbosity is specified not via a function keywords arguments, but via an instance attribute e.g. self.verbose (or, pre-conversion, self.info). To manage these I have created another decorator which is analagous to the existing _manage_log_level_via_verbosity but adapts the log level for runs of decorated methods according to the value of the verbose as an attribute instead as a keyword argument.

Work still TODO for this PR

Currently a WIP with functionality largely there but docs & testing pending:

• apply deprecation messages on info & _debug where appropriate;
• adapt test verbose specifications;
• add docs section;
• adapt docstrings;
• double check format conversions from multi-argument print statements;
• add explicit assetLog-based tests;
• manual testing in anger (esp. check interfacing with cfdm logging);
• add new function to API ref;
• update changelog;
• address DH feedback on docstring formatting.