MINOR: sample: converter: Add add_item convertor

MINOR: sample: converter: Add add_item convertor

This new converter is similar to the concat converter and can be used to
build new variables made of a succession of other variables but the main
difference is that it does the checks if adding a delimiter makes sense as
wouldn't be the case if e.g the current input sample is empty. That
situation would require 2 separate rules using concat converter where the
first rule would have to check if the current sample string is empty before
adding a delimiter. This resolves GitHub Issue #1621.