Fix threading test that sporadically fails on Travis.

threading.active_count will return the number of threads that would be
returned by threading.enumerate. These may include additional worker
threads that are unrelated to the user-spawned threads, and would cause
the loop to continue to wait even though all user-spawned threads were
totally complete.
add missing symbol curr_file to .def file
More resilient StationXML parser.

It now pops a warning when it cannot convert some string to a float or
the float is a NaN. It does not stop parsing anymore. This is
unfortunately needed for some real data out there.