Add ability to request final state for any line

Resolves #57.

Defaults to unchanged from previous line, which is the prior behavior.

When setting this via Update Line Final State.vi, it checks whether or not
that line's final state has already been set to an output of a different
kind.

The options are High, Low, or Same, which are self-explanatory, and also
Default, which should not be used by the user.  Default has the behavior
of Same, but is used internally to distinguish from an entry explicitly
set to Same vs. something unchanged from the default.
Added "Final State" to Slave_Line test device

For use in testing the new functionality (it worked) as well as a very
simple demo of how to implement things for other devices.