Improvements to documentation of icarMilkingVisitEventResource

[cookeac]

Further to the discussion on #436 we would like to improve the documentation for icarMilkingVisitEventResource as follows:

1. Add a description to milkingMillkWeight to clarify that this is the milk weight per the ICAR Guidelines (i.e. a "certified" milk weight).
2. Improve the description of characteristic in icarMilkCharacteristicsType by creating an enumeration file of the values and updating the description to specify that the values of the enumeration should be used.