Adds test_data to .gitignore

Changes to files in test_data must be made forcefully.
Fixes read-only support

Addresses a couple spots where opening the coverage in read-only ('r') mode wasn't being respected properly.

Addresses [OOIION-664](https://jira.oceanobservatories.org/tasks/browse/OOIION-664)
Merge pull request #12 from blazetopher/mode_fix

Mode fix