Add sortmerna to assign_taxonomy.py

Partially resolves https://github.com/gregcaporaso/short-read-tax-assignment/issues/68.

Most of the functions follow those in https://github.com/ekopylova/short-read-tax-assignment, with modifications relating to Null alignment strings now output by sortmerna.

Could you move this method to the ``TaxonAssigner`` base class, rather than duplicating the code here?