Nicking enzyme - support for Dseqrecord and Dseq

[manulera]

nicking enzyme always returns a Dseq, but it would be better I think if it could return a Dseqrecord keeping its original features. It could also work for Dseq as inputs.

You could check whether the argument passed is child of Dseqrecord or Dseq and do differently for each. Alternatively, you could include them as methods of the classes as I mention in #18.