An node from an RNA template is either a sequence of non-paired bases,
a base pair originally belonging to an helix,
or a single base originally belonging to an helix but which was broken
in order to remove pseudoknots.
For an helix, tells us whether IN1/OUT1 is the 5' strand
(the first strand we meet if we follow the RNA sequence)
or the 3' strand (the second we meet if we follow the RNA sequence).