Logging enhancements

[sadielbartholomew]

Suggestions for extensions to the initial work merged in the PR #82. Equivalent to NCAS-CMS/cfdm#35 but for cf; for more details for each point here cross-reference with the points in the same order under that Issue:

• new meaningful messages across the codebase & verbose kwargs to functions as applicable;
• improved display in log calls of objects for readability;
• add a file handler to optionally log messages and user calls with timestamps to file(s).

In this case there is also the matter of the interrelation between the logging emerging directly from cf & that originating from cfdm via import, but I will include that as part of a separate GH Issue as there are other global settings in both libraries which should be examined in that respect.