[TangerineSDR] Tweaking rubidium

[Phil Karn]

After sitting all night, it's a phase change of -9 degrees in ~1000 
seconds, or a relative error of -2.7e-12. Not bad.

Next step is to implement that software calibration scheme I mentioned, 
where a channel uses its software PLL to adjust the calibration on the 
RX888's own TCXO. Will be useful for those without hardware references 
or those who don't want to semi-permanently require the use of one in 
their RX888.

I'm going to have to be careful lest I become a time nut.

Phil