Informative version change information on variables

[martinjuckes]

When a record changes, it should be possible to record some information about the change.

(1) A date, or request version, of the last change.
(2) A PEP encoding of the change: major - minor - patch, with an approach that allows automated decisions about the kind of increment involved. This is only likely to work for mature variables. Others could just be labelled as 1.0bnn, with nn incrementing at each change.

• major : change of label (short name), units, dimensions, coordinates, standard name (other than standard name aliases), cell methods (other than comment string);
• minor : revision of title (long name) and description which clarifies the intended nature of the variable
• patch : revision of title or description to improve language or consistency, replacing standard name with alias, change in comment or usage notes.