pile_viewer bugfix: If a marker's event didn't have a name (None) goto failed

    Traceback (most recent call last):
      File "/usr/local/lib/python2.7/dist-packages/pyrocko/pile_viewer.py", line 3079, in inputline_returnpressed
        clearit, hideit, error = self.viewer.inputline_finished(line)
      File "/usr/local/lib/python2.7/dist-packages/pyrocko/pile_viewer.py", line 2970, in inputline_finished
        self.go_to_event_by_name(arg)
      File "/usr/local/lib/python2.7/dist-packages/pyrocko/pile_viewer.py", line 1987, in go_to_event_by_name
        if event.name.lower() == name.lower():
    AttributeError: 'NoneType' object has no attribute 'lower'
extended some documentation
Merge branch 'uplink' of https://github.com/HerrMuellerluedenscheid/pyrocko