Add MultipleTreatment concept to Resources

[dibley1973]

It is acceptable when...

• The concept is developed within it's own feature branch
• The concept will be represented by an object
• The object derives from a Entity
• The object has an Duration property
• The object has a Technician property
• The object has a Cost property of type Money
• The object can contain multiple "sub" single or other MultipleTreatment objects
  -- A discount of cost can be applied to the total of the sub treatments
  -- A discount of time can be applied to the total of the sub treatments
• The object has a static factory Create() method
• The Create() method, and properties are covered with units tests with at least 90% code coverage