Clarification of Python and YAML files on default options

At the moment we have the 3 following files relating to default options:

From a git grep -i default in the DaVinci repository I only see DaVinci.options_default.py as being read/imported (needed by python/DaVinci/application.py to define options). I have the impression that there's some level of redundancy and/or obsolescence, whose clarification is the purpose of this issue, to proceed with a clean-up as adequate.