Generate data interface generated by magento2:generate:crud_model in Api/Data

[adamonsoon]

M2 documentation states that we should

Define data interfaces in the Api/Data subdirectory for a module.

At the moment it's generated under Model/ThingInterface.php

I could fix send a pull request by tomorrow if it's actually an issue.

[astorm]

@adamonsoon Where's that stated?

[adamonsoon]

http://devdocs.magento.com/guides/v2.1//extension-dev-guide/service-contracts/design-patterns.html#data-interfaces

[astorm]

Thanks @adamonsoon! I did a spot check on Magento's core and that seems legit. A pull request would be welcome.