output/csv: check unsupported/untested input signal conditions

output/csv: check unsupported/untested input signal conditions

The current implementation of the CSV output module makes assumptions
which don't hold. Which results in incorrect or incomplete output for
some combinations of logic and analog signals.

Check for some of the known problematic conditions, and warn the user
about potentially unexpected results. This is a workaround until the
issues properly get addressed in the implementation.

This is motivated by but does not resolve bug #1026.