add TSV to VCF script to iVar Titan container

[rpetit3]

This PR replaces #224 (see for previous discussion) so as to only affect the ivar-titan container, not the main one.

The PR adds a script, ivar_variants_to_vcf.py, to convert the iVar variants in TSV format to VCF. The script is pulled from nf-core/viralrecon v2.1 release.

ivar_variants_to_vcf.py had /user/bin/env python in it's shebang line, the container has python3 in the path but not python. Rather than editing ivar_variants_to_vcf.py, I used update-alternatives to add python (just a symlink to python3).

Please let me know if there are any questions, or anything needed on my end.

Example Usage

docker run --rm -u $(id -u):$(id -g) -v ${PWD}:/data ivar:1.3.1-titan ivar_variants_to_vcf.py 20154160_S5_L001.variants.tsv 20154160_S5_L001.variants.vcf

Attaching inputs/outputs (w/ .txt extension to make GitHub happy)
20154160_S5_L001.variants.vcf.txt
20154160_S5_L001.variants.tsv.txt

[kapsakcj]

Looks good, thanks for adjusting so that this affects the ivar_titan dockerfile.

I'll go ahead and merge, re-build, and push this image to dockerhub and quay.

[rpetit3]

Thank you!