Given a sequence greater than 8 bp oriented 5' -> 3' and a seed
definition, this function will return an object containing seed-specific sequence
information. Users can input a custom seed name, but must provide the start
position (start.pos) and stop position (stop.pos) that define the
range of the seed sequence.
Built-in options: mer8, mer7A1, mer7m8, mer6
Note: The seed definitions mer8 and mer7A1 force a U at position g1.
This results in an A in the target sequence being searched.