Consider moving cloning functions to Dseq and Dseqrecord

[manulera]

Hi @hiyama341,

I think some of the functions in the cloning module, such as USER_enzyme and nicking_enzyme could be included into pydna as methods of Dseq and Dseqrecord. Only if you think that's a good idea, of course.

There is some functionality for cas9 in pydna, but not sure it does exactly the same as what you have there.

cc @BjornFJohansson