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 prefer to use context managers (and they're recommended):
```
with open(fasta_tempfile_name, 'w') as f:
    f.write(fasta_seqs)
```