ocrd-tool.json: Support range/step/unit definition for numerical parameters

[kba]

For those implementations that want to generate user interfaces for the parameterization of processors, more information about numerical parameters would be helpful to offer the right widgets in the UI. This includes

• range information (what is the minimum and maximum of supported values)
• step size (are these integer values to be implemented with a simple counter or even a dropdown or floating point values to be represented with e.g. a slider)
• unit (e.g. are these rotational degrees or foreground-to-background-pixel ratios or distances etc.)

[kba]

With #206, range can expressed with {exclusiveM,m}{ini,axi}mum.