Switch NTC power

As wired, any time we have power to the Teensy, the NTCs will be powered.

Initially I thought the 5V supply from the vehicle went away a while with key off, but I am no longer sure. So it is best to make the NTCs switchable, to avoid leakage through the NTCs when the vehicle is off.

I can either switch the supply voltage, or possibly switch the ground (which would be easier), but I need to verify the ground for the NTCs is truly independent from the rest of the vehicle first. Not just check the schematics, use my meter and verify physically!

I might want to read the NTC reference (high or low side, whatever I end up switching) for increased accuracy. Or, since I can't find specs on these NTCs, and even GM just appears to view them as ADC counts, maybe accuracy is overrated...