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.
Can you please write out the forward and reverse sequences (in the same order, with same IDs) in two separate files? This way users can join them with e.g., join_paired_ends.py.