Finished multi-sensor board

Completed multi-sensor board.  Supports the following sensors:
- SM5740 Differential pressure sensor
- 3 voltages from 0-30V referenced to board ground
- 1 high precision differential voltage with common mode between 0.6-1.5V
- serial link (5V compatible via open drain buffers)
- 1 shunt current meter (0-15A) either lowside or highside with a ridiculous Common Mode voltage range (LT1990 is the op-amp front-end for level shifting)
- pulse counting with 2.8V voltage source and open drain compatible input for turbine flow sensing

All of the analog circuitry is based around a precision AN8557 digitally controllable instrumentation amplifier.  This means that you can program in the gain and input offset correction in the field on the fly.  This means the part can retarget many different types of analog readings and can automatically calibrate itself.  Two analog multiplexers are used to allow switching the input polarity so that the circuit can automatically detect input offset errors and null them out.   

BOM cost comes in at $12 for 30 units because the op-amps are high quality.