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.
Minor comment, but I find this form to be a bit easier to read and work with:

```python
with self.assertRaises(SeqLengthMismatchError):
    get_consensus(fasta_file_for_consensus_unequal_length, 2)
```

Completely up to you though