access by scan number
Must define self.import_file as file name.  Also work on making better choices for starting directory in open/save file dialogs
begin integrating the fallback SPEC file reader
refs #7, still unresolved "KeyError: u'tree not in affiliation'" at:

  Traceback (most recent call last):
    File "C:\Users\Pete\Documents\eclipse\nexpy\src\nexpy\gui\treeview.py", line 389, in selectionChanged
      self.statusmessage(node)
    File "C:\Users\Pete\Documents\eclipse\nexpy\src\nexpy\gui\treeview.py", line 365, in statusmessage
      text = message.tree
    File "C:\Users\Pete\Documents\eclipse\nexpy\src\nexpy\api\nexus\tree.py", line 1349, in __getattr__
      raise KeyError(name+" not in "+self.nxname)
  KeyError: u'tree not in affiliation'