Refactor DataType.getPath

[dlindhol]

Instead of relying on flattening the model, which introduces the need for dots (".") in identifiers, find a more functional routine to traverse the graph of the model to map a given variable to its value in a Sample.

Note that the Scalars and nested Functions within the DataType (a.k.a. model) graph can be logically zipped with the Data values of a Sample. (Tuples map to the value representing the first element.) The goal of getPath is to provide a mapping from a given variable identifier to its location in a Sample. The implementation using SamplePath is open for redesign.