WaveformData Fixes

[jarom77]

This fixes two errors in oscope_scpi/keysight.py:waveformData.

No Waveform Data

If there is no waveform data (scope hasn't been triggered yet, etc), WAVeform:PREamble? returns 22 values rather than 24. This causes the next line to throw an unhelpful error. While the error from the oscope is helpful, a ValueError exception is a mischaracterization of the error, so catching it doesn't make sense. I rewrote it to throw a BufferError instead (line 793-796) so it can be more intuitively caught and handled.

ERROR(00): 41,"Waveform data is not valid.", command: ':WAVeform:PREamble?'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "oscope_scpi/oscilloscope.py", line 329, in waveform
    (x, y, header, meta) = self.waveformData(channel, points)
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "oscope_scpi/keysight.py", line 799, in waveformData
    (x, y, header, meta) = self._waveformDataNew(chan_str, points)
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "oscope_scpi/keysight.py", line 401, in _waveformDataNew
    (wav_form, acq_type, wfmpts, avgcnt, x_increment, x_origin,
ValueError: not enough values to unpack (expected 24, got 22)

oscilloscope.py:waveform could also be rewritten to just return None or 0 on this error, but I felt preserving the exception provides better control to the user.

Channel Argument

waveformData says it takes channel as a string or None. When None is passed, we use self.channel, which isn't necessarily a string. Now it can take either a string or an integer because conversion happens within the function.

I also moved setting the waveform source to waveformData and out of the called functions _waveformDataNew and _waveformDataLegacy. It needs to be in waveformData in order to query the waveform completeness (line 794). We could leave setting this in the called functions, but it would be redundant.