toXML() methods implemented for Set, Cell and Part.  Sets can construct and return an unindented xml String.  Could benefit from formatting.
Created CellDirectory class which extends File class.  CellDirectories are identical to Files except that a CellDirectory's toString() returns the relative file path and not the complete file path.  CellDirectory objects are now added to the tree nodes instead of Files so that the tree does not display the entire file path.
Forgot to track CellDirectory class
Forgot to track CellDirectory class