Allow string probes to handle networks

The string syntax 'A.B' could mean to probe signal B on object
A, or to do a default probe on the object 'A.B'.  The fix here
is to check if 'A.B' exists, and if it doesn't, then try looking
for 'A' and probing it for 'B'.
Made PassthroughNodes probeable
Changed BG "Input" to "input" and "Output" to "output"
Added BG test