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.
I think the order on this tuple is (Example title, Description, command) It looks like you've title and description on the same one (and on description only have `output_dir`, which I'm assuming is going to look weird in the actual output