Fix error when selecting large ranges in Interpretation

[jopohl]

Fix errors like this:

[ERROR::SignalFrame.py::update_number_selected_samples] ndarray is not C-contiguous
Traceback (most recent call last):
  File "/home/andy/work/repository/urh/src/urh/controller/widgets/SignalFrame.py", line 320, in update_number_selected_samples
    power = np.mean(self.signal.iq_array.subarray(start, end, step_size).magnitudes_normalized)
  File "/home/andy/work/repository/urh/src/urh/signalprocessing/IQArray.py", line 86, in magnitudes_normalized
    return self.magnitudes / np.sqrt(self.maximum**2.0 + self.minimum**2.0)
  File "/home/andy/work/repository/urh/src/urh/signalprocessing/IQArray.py", line 82, in magnitudes
    return get_magnitudes(self.__data)
  File "src/urh/cythonext/util.pyx", line 128, in urh.cythonext.util.__pyx_fuse_0get_magnitudes
  File "stringsource", line 660, in View.MemoryView.memoryview_cwrapper
  File "stringsource", line 350, in View.MemoryView.memoryview.__cinit__
ValueError: ndarray is not C-contiguous

when selecting larger ranges of a Signal in Intepretation.