Provide explicit flag to allow overwriting of output files

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[x] Make sure you are requesting to pull a topic/feature/bugfix branch (right side). Don't request your master!
[x] Make sure you are making a pull request against the develop branch (left side). Also you should start your branch off our develop.
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Pull request type

Please check the type of change your PR introduces: - [ ] Bugfix - [x] Feature - [ ] Code style update (formatting, renaming) - [ ] Refactoring (no functional changes, no api changes) - [ ] Build related changes - [ ] Documentation content changes - [ ] Other (please describe): ## What is the current behavior?

When attempting to test, it can be a nuisance to have to target a new output location for repeated runs. In the present code base, failing to specify an unused output location results in failures to write.

Issue Number: N/A

What is the new behavior?

The new behavior allows the specification of an --overwrite flag that will allow successive test runs to re-use output locations.

Does this introduce a breaking change?

[ ] Yes
[x] No

wri / gfw_forest_loss_geotrellis