Added an Arduino controlled Si5351 VFO to the Bitx40

Like, it seems, everyone on the internet, I have added a digital VFO to the Bitx40 board.

The Bitx40 board is a great base for experimenting with a 40m QRP SSB radio.

The code for the Arduino is based on code from AK2B who based it on code from Jason Mildrum, NT7S and Przemek Sadowski, SQ9NJE.

I slightly modified it to use the Etherkit version of the Si5351 library (which can be installed from within the Arduino Library manager).

Other changes were to strip out some things not needed for the Bitx40, enforce 40m band edges and improve the display of the step size a bit. Here's a video of it in use receiving.

I did run into insufficient space on one of the Arduino I had in the junk box but another worked fine. My fork of the code is available on github here. Having done all this, I'm now having second thoughts and am considering doing what Peter VK3YE did and using a ceramic resonator for the VFO.