PR02 low-power thermal step

Read pr02-resistor-warmup-cooldown.html before assembling the bench.
R1 is the Vishay PR02000201000JR500. Use 3.30 V and a verified 35 mA
current limit. Measure the PR02 voltage and the voltage across a
10.00 Ω sense resistor with separate floating voltage meters. Attach
the insulated Type K probes to the coupon undersides. Keep the
mounting and reference fixed through heating and cooling.

Copy observations-blank.csv and enter actual sample times and values.
Analyze the completed file with, for example:
    python3 thermal_step.py observations.csv --on-s 0 --off-s 175

Operate the source and the manual stops independently of the analyzer.
The coupon trace is specific to the mount and probe placement.

Host tests use constructed values:
    python3 -m unittest -v
