add resource schema and resource_locations to ocrd-tool.json schema

[kba]

Adds two new properties to the tools list items in ocrd-tool.json:

• resources: List resources that can be used with this processor, to be used with OCR-D/core's ocrd resmgr CLI and resource lookup mechanism
• resource_locations: By default, resource's are looked up in data ($XDG_DATA_HOME), cwd and system (/usr/local/share/ocrd-resources). Since not all processors can support the full range of locations (notably ocrd_tesserocr can look only in one location for models), this allows restricting the locations to look for this processor.

Until now, we're maintaining a central resource registry in OCR-D/core. We want to decentralize this, allowing processor developers to list resources in their own ocrd-tool.json.

[bertsky]

Should probably be addressed here, too:

• resources of type directory (see discussion about possible representation in parameters)
• resources that reside in the distribution directory of the module and come pre-installed (think Python site-packages).

[bertsky]

• resources that reside in the distribution directory of the module and come pre-installed (think Python site-packages).

there should be no need to register these, though

[bertsky]

Also, the ocrd_tool.md needs to be updated as well:

• mention the resources and resource_locations under the File parameters section
• show them in the examples