Improve example outputs

[WizardTim]

• Changed naming of 'PSU' and 'Load' voltages to 'VIN+' and 'VIN-' as I initially thought 'PSU' was referring to I2C logic level Vcc. I can't find any IN219 eval boards that don't used VIN+/VIN-. TI datasheet uses 'bus' and 'load' terminology in some circuit diagrams which might have influenced the choice?
• Added power figures both from on chip register (lower resolution) as well as one calculated in Python using the full resolution voltage/current values
• Align output in columns with correct significant figures
• Added prompt if internal math calculations result in an overflow

Old

PSU Voltage:    3.292 V
Shunt Voltage:  0.003520 V
Load Voltage:   3.288 V
Current:        0.035200 A

New

Voltage (VIN+) :  3.292   V
Voltage (VIN-) :  3.288   V
Shunt Voltage  :  0.00352 V
Shunt Current  :  0.0352  A
Power Calc.    :  0.11574 W
Power Register :  0.115   W

With gain = DIV_1_40MV and _cal_value = 65535 to force a math overflow in current register.
New

Voltage (VIN+) :  3.128   V
Voltage (VIN-) :  3.088   V
Shunt Voltage  :  0.04000 V
Shunt Current  :  3.2767  A
Power Calc.    : 10.11845 W
Power Register : 10.118   W

Internal Math Overflow Detected!

[siddacious]

@WizardTim works great, thanks for the improvements