Zero is no good: we take the log of values, and if they're zero that explodes.
For Norovirus the issue was that we look at the I/II ratio and if either of those is zero then we'll emit a zero for that pathogen; in that case we can fall back to the annual average ratio.
For Influenza the issue was that some weeks had zero positive tests; in that case we somewhat unprincipledly treat it as if there were really 0.1 positive tests.
Zero is no good: we take the log of values, and if they're zero that explodes.
For Norovirus the issue was that we look at the I/II ratio and if either of those is zero then we'll emit a zero for that pathogen; in that case we can fall back to the annual average ratio.
For Influenza the issue was that some weeks had zero positive tests; in that case we somewhat unprincipledly treat it as if there were really 0.1 positive tests.
Fixes #148, with a test to verify it stays fixed.