public interface TaskInputExpression
  A TaskInputExpression is a String prefixed with either "$input" or "$inputRecord",
  followed by an optional dot-separated hierarchical path, which can specify either an
  atomic data value (String, Number) or Record from the input data.  For example, if the
 Process.state represented in JSON
 were:
  
  {
     orderId:5,
     orderItems: [
        {name:"Pencils", quantity:3, itemId:2344}
     ],
     orderUser: { name:"Henry Winkle", address:"...", ... }
  }
  
  .. and a task specified an inputField of "orderId" and an inputFieldList of
  "orderItems","orderUser", then: