skip: unrecoverably mismatched region length

[elija-feigl]

Issue: Bug in Cadnano_oxDNA.py: Cadnano to oxDNA converter. the Cadnano file has to have the following motifs:

• square lattice
• a double-stranded segment ending with a scaffold 1 base longer than the staple
  • a skip occurring later in that helix --> critical error raised the error disappears:
• without the skip
• if the scaffold overhang is longer than 1 or 0 (also see minimal example)

minimal example: cadnano-tacox-mismatch.json.txt

python cadnano_oxDNA.py cadnano-tacox-mismatch.json sq

"CRITICAL: the lengths of begin (2) and end (3) arrays are unrecoverably mismatched"

[lorenzo-rovigatti]

The bug has been fixed by @mckwxp