Single-phase energy meter

I did some testing to verify that samples are correctly logged into the FLASH memory. I've been able to read them back through USB and plot some of them in excel. Plots show power consumption of my Ender 3 Pro 3D printer as well as voltage variation during the print (logging period is 2s).

RF communication is finally working. It wasn't easy to get those cheap chinese RF modules working but this library helped me a lot.

Two new commands have been added:
--RFCH<xxx> - set RF channel (0-127)
--AD<aaaaaaaaaa> - set 5-byte address for RF communication (HEX)
These two commands are also used for setting the USB dongle on PC side, which must have the same channel and address as energy meter you want communicate with.