added utility methods to the FileSet model

- $fileset->getFiles() returns an array of File objects for a given set
- two static functions to get the files directly via fsID or fsName
updated new retrieval methods to call self instead of FileSet for best practice
updated getByID and getByName methods to be static
Merge pull request #1069 from aghouseh/fileset-methods

added utility methods to the FileSet model to return relevant File objects