DS1054Z specific version of Spectrum Analyzer for the Rigol DS1000 series digital scopes

First and foremost many thanks to rheslip and ppmkm for the original work.

Changes to rheslip's and ppmkm's version:

1. PyVISA version 1.10.1
2. Tested under Linux (Mint).
3. pyvisa-py used.
4. python3 used.

Installation instructions for Linux (2021-03-15):

1. Install following software:
   • pyvisa (pip)
   • pyvisa-py (pip)
   • pyusb (pip)
   • numpy (pip)
   • scipy (pip)
   • tkinter (sudo apt install python-tk)

Original README.md by rheslip is in ORIGREADME.md and edited by ppmkm is in README_ppmkm.md