new traces from scratch did not have self.file

Hi,

this snippet:

    import numpy as np
    from pyrocko import trace, pile

    ydata = np.array([1,2,3,4,5,51,2,3,2,2])
    tt = trace.Trace(station='test', ydata=ydata)
    tp = pile.Pile()
    tp.add(tt)
    print tp.all()


caused the following exception:

    Traceback (most recent call last):
      File "test_kann_weg.py", line 8, in <module>
        print tp.all()
      File "/home/zmaw/u254061/.local/lib/python2.6/site-packages/pyrocko/pile.py", line 1028, in all
        for traces in self.chopper( *args, **kwargs ):
      File "/home/zmaw/u254061/.local/lib/python2.6/site-packages/pyrocko/pile.py", line 995, in chopper
        chopped, used_files = self.chop(wmin-tpad, wmax+tpad, group_selector, trace_selector, snap, include_last, load_data) 
      File "/home/zmaw/u254061/.local/lib/python2.6/site-packages/pyrocko/pile.py", line 901, in chop
        if tr.file not in used_files:
    AttributeError: 'Trace' object has no attribute 'file'

The two extra lines should avoid this.