PLCgen: I/O table CSV file 4-cell line confusingly requires only 4 cell-lines

Copied from #897 (comment 2845644)

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This feature doesn't work in v5.x. I have tested it with io.cif from plcgen-test-models-v27 and version nightly-20241023-115319. In this test I have used the following io table:

%I1.0,,in_bool,mijn-Bool
%ID2,,in_int
%ID6,,in_real
%Q1.0,,a.out_bool
%Q1.1,,a.out_alg_bool
%QD2,,a.out_int
%QD6,,a.out_real

... and I have received the following error:

Addresses #679