cmpe.py: Add SI unit conversion function

Support SI unit conversions via unit(value, input unit, output unit).
For example, unit(0.5, 'k', 'c') returns 50000.0.  A blank unit indicates
10**0, so unit(0.5, 'k') returns 500.0.

Signed-off-by: Nate Case <nacase@gmail.com>
cmpe.py: Add parallel resistance function rpar()

Shorthand for 1 / (1.0/r1 + 1.0/r2) used for calculating equivalent
resistance for r1 and r2 in parallel.

Signed-off-by: Nate Case <nacase@gmail.com>