First on/off sine wave generator

Whee, it's alive.
tmcpoke can turn the waveform on and off, but there's no frequency
control, and there's still something wrong with the scpi interface.  It
worked at home, but not at work?!
Fix timing to get expected output frequencies
Add APPL:SIN [freq] support

At least for a reasonable range.  Need to be maths to work out the full
range now allowed.

Still having usbtmc overflow problems on this computer!