Provide new script for default SSU database download and build

We have overhauled the way we download and build a cleaned SILVA SSU database for EMIRGE. SILVA is the recommended small subunit rRNA gene database to use for EMIRGE. This is back-ported from Elmar Pruesse's EMIRGE2 code, and replaces the old emirge_download_candidate_db.py with the new emirge_makedb.py.

This new script requires vsearch, as EMIRGE2 is moving to using vsearch in place of usearch.

Maintenance Release

Fixes compatibility with BioPython >= 1.67

Fix missing zlib dependency

fixes issue #23

Release to pypi

v0.60.2

Bump version: 0.60.3 → 0.60.2

bugfix release

• fixes compatibility issue with pysam >= 0.8.4
• fixes providing pre-computed mapping file (issue #19)