IOGroup controls that return NAN cause issues during restore

[bgeltz]

Describe the bug
I tried to write an IOGroup that returns NAN for some controls and I expected no issues during tear down instead an exception was thrown out of PlatformIO.adjust because of the NAN.

GEOPM version
333929d

Expected behavior
If a NAN was saved, it should be ignored during restore.
Or make it so NAN's are illegal as return values from read_signal().

Actual behavior
An exception was thrown during restore.

Error messages produced
TBD

Additional context
Related to these workarounds that went into the IOGroup tutorial:
#3410