Default values for inputs

[dpshelio]

This is more of a hypothesis. What to do if the yaml file is incomplete? (i.e. some lines missing, or some entries are not provided?)

• do we fail?
• do we use default values for these lines?

purify -g <filename> could be used as in doxygen to generate a template for the config file.

[RolandMacDoland]

The YamlParser must have a write method to generate a default param.yaml file.