Filter in obspy.signal.cross_correlation.xcorrPickCorrection silently modifies arguments

The documentation for `obspy.signal.cross_correlation.xcorrPickCorrection` indicates that one might use the `filter` argument to change the processing slightly. What's _not_ mentioned is that this filtering is done on the input traces themselves.

Personally, I think `xcorrPickCorrection` should filter on a copy, but if this is intended, it should be documented as it's not something one would expect.