Add cc0 license-maven-plugin configuration

[hinerm]

Tried running mvn license:update-file-header and got BSD instead of cc0. All the projects should be updated to be cc0

[ctrueden]

This will require either:

• A pull request to the license-maven-plugin adding CC0 as a built-in option; or
• A custom license server providing that option.

We have done the custom license server in the past, and it has some problems. So I put in effort to switch us fully over to built-in options. It is probably worth pursuing the PR option to keep things that way.

[ctrueden]

Note that instead of CC0 we could use the Unlicense; the latter is what choosealicense.org recommends for disclaiming copyright. Either way.

[hinerm]

You can just provide the license in-repo and reference it via the maven config.. see scifio-tutorials

[ctrueden]

There is some reason that doing that caused problems, too. Sorry I can't remember the details now... Generally speaking, you want to avoid in-repo file:// references in the POM (see item 6 of the Playing Tradeoffs with Maven article). But in this case maybe it is OK because the license-maven-plugin config is not required for the normal build lifecycle.

Anyway, if we are using CC0 (or Unlicense) in multiple places, I would still favor submitting a PR upstream, even if we use a local license server in the meantime.