Significant performance improvement in Tektronix5014 driver

[sldesnoo-Delft]

Changes proposed in this pull request:

• For the conversion of 4 waveforms with 1e6 Sa the time is reduced from 890 ms to 310 ms.
• The change in _pack_waveform gives a reduction of 180 ms.
• The change in _pack_record gives a reduction of 400 ms.

[codecov]

Codecov Report

Merging #2910 (8c16444) into master (1bc4723) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2910   +/-   ##
=======================================
  Coverage   65.31%   65.31%           
=======================================
  Files         209      209           
  Lines       28037    28037           
=======================================
  Hits        18312    18312           
  Misses       9725     9725           

[astafan8]

@jenshnielsen i think this looks good now, shall we merge?