Sortmerna api

App controller + accompanying unit tests
It worked on my end:
```
n [1]: sortmerna_db = "foo"

In [2]: s = set(sortmerna_db + ext for ext in ['.bursttrie_0.dat', '.kmer_0.dat', '.pos_0.dat', '.stats'])

In [3]: s
Out[3]: {'foo.bursttrie_0.dat', 'foo.kmer_0.dat', 'foo.pos_0.dat', 'foo.stats'}
```