[ddlm-group] Dictionary versioning

The vagueness of dictionary versioning has been raised as an issue (see https://github.com/COMCIFS/cif_core/issues/47). Now that dictionaries can import template dictionaries, it becomes possible that the template dictionary could change in ways that would render the main dictionary incorrect, for example, if a necessary attribute was removed from the template definition.  Such fiddling with template attributes has recently been proposed (see https://github.com/COMCIFS/cif_core/issues/42) as a solution to certain technical issues.

While COMCIFS will obviously endeavour to maintain both template dictionaries and main dictionaries as a compatible whole, we should come up with some principles for versioning to guide authors and editors, as well as authors of dictionary checking software.  I suggest that we use semantic versioning of the form <major>.<minor>.<patch>, where a change in the major version number is required when incompatible changes are introduced.

There are two situations that are important: importing pieces of a definition from a template dictionary, and importing a whole dictionary in order to build on it. 

Versioning in template dictionaries: Firstly, there has been no explicit statement of how importation should treat the presence of the same attribute in both the template and the importing definition - I suggest the simple principle that the value in the importing definition always has precedence over the imported value. Assuming this, a template dictionary will be potentially incompatible with an importing dictionary if attributes are removed from a definition, a definition is itself removed, or the value of an attribute is changed in a way that would change the behaviour of software.  Either of these three changes would require an increase in the major version number of a template dictionary.  Other changes are covered by the rules below for full dictionaries.

Versioning in full dictionaries: we never make any changes in a domain dictionary that would require a change in major version number as this would undermine our goal of stable, universal data names. We are then left with simple rules for changing the non-major version numbers in both full and template dictionaries:

1. change the patch version for typo correction, rewording and clarification
2. increment the minor version for all other changes: additions to enumerations, new definitions, moving data names to aliases of new definitions

Feel free to respond either on the github issue or here.