Instead of '?', set default gene_coverage to 0

[j23414]

Description of proposed changes

After discussion, it became clear that keeping gene_coverage columns a numeric value may simplify the subsequent augur filter command.

Related issue(s)

• https://github.com/nextstrain/dengue/pull/18

Checklist

• [x] Checks pass

[j23414]

Went ahead and merged, since this arose from discussion and is a minor change.

Although feel free to submit post-merge comments.