v2.1.0

1. add task centrifuge
2. centrifuge can be run as a standalone wdl workflow as well: wf_centrifuge.wdl
3. modify bbduk wdl task to decontaminate for Ecoli and dermacoccus provided by new bbduk contamination database
4. switch to gatk 4.5.0.0: includes hardware acceleration and is significantly faster, changes described in https://github.com/broadinstitute/gatk/releases/tag/4.5.0.0
5. switch to dbest/varpipe4:v2.0.3: includes fix to coverage calculation described in CDCgov/NCHHSTP-DTBE-Varpipe-WGS#16
6. switch to the newest version for multiQC: v1.19
7. add standalone kraken2 wdl workflow: wf_kraken2.wdl