Add logging to manage & configure verbose flag logic

[sadielbartholomew]

Equivalent to NCAS-CMS/cf-python#37. We have decided to add logging here (to cfdm) first as cf-python builds on top of it so as a base it is a more natural starting point, & there is less code to cover to fully implement the logging as a POC for possible adjustment.

We have also decided the logging should be promoted as a user feature for configurable feedback, not just as a developer aid.

We will go with Python's standard logging module as it is excellent & certainly sufficient for the requirements here.