addition of LEA-Seq method

Implements Low-Error Amplicon Sequencing (LEA-Seq) method, described in: Faith, Jeremiah J., et al. "The long-term stability of the human gut microbiota." Science 341.6141 (2013).
I need to add a couple of functions in split_libraries_lea_seq.py, and I need to add unit tests. I will add those soon.
Why assigning the parameter to an internal variable? Just use the parameter and/or rename the parameter if you think it is not explanatory enough. I think this is happening in some other functions, can you also fix that on them?