Convert Condition to YAML on demand

This MR changes the way (DetDesc) Condition objects can be accessed as YAML::Node: instead of making a YAML version at construction time (when parsing XML) the conversion is done the first time it is needed.

This closes #241 (closed) and Moore#473 (closed) by avoiding the waste of space due to unneeded YAML copies of the conditions.

As a side note, I added a couple of missing includes.