Update Visit to include VisitAttributes

Add to the Visit class:

• property, plus getter and setter: List<VisitAttribute> attributes
• method: addAttribute(VisitAttribute) // exception if the attribute type specifies uniqueness and exists already
• method: setAttribute(VisitAttribute) // overwrites any existing value for that type
• method: List<VisitAttribute> getAttributes(VisitAttributeType)

Add to the full-form getVisits(...) method in VisitService:

• List<Visit> getVisits(..., Map<VisitAttributeType, Object> attributeValues, boolean)
  • this would limit the returned visits to only those with all of the attribute type -> values in the map

Extend the VisitValidator to check for minOccurs, maxOccurs, and required on its visit attributes.

TODO: is this change to the full-form getVisits(...) method right?