CI tests give deprecation warnings

[peendebak]

scipy is generating deprecated warnings in the CI testing. See for example https://app.travis-ci.com/github/QuTech-Delft/qtt/jobs/558612837

src/tests/integration/algorithms/test_generic.py: 34 warnings
  /home/travis/build/QuTech-Delft/qtt/src/qtt/algorithms/generic.py:573: DeprecationWarning: Please use `convolve` from the `scipy.ndimage` namespace, the `scipy.ndimage.filters` namespace is deprecated.
    filtered_signal = scipy.ndimage.filters.convolve(filtered_signal, boxcar_kernel, mode='nearest')

@QFer