Support @Parameter annotation for Ops (in scripts)

It would be great to be able to use ops with the `@Parameter` annotation and as script parameter, as pointed out in a [forum thread](http://forum.imagej.net/t/script-parameters/3362/3?u=stelfrich).

Something like:
```groovy
// @Threshold(label="What Threshold to use?") threshold
// @Img input
// @OUTPUT Img output
output = threshold.compute(img)
```

I guess this requires establishing a connection between the requested op and input to enable matching?

Start with the linked forum thread and the Groovy script example in the issue to understand the requested @Parameter and script-parameter behavior. Done means Ops can be requested through the annotation, matched to the script input, and used as shown in the example.

Written by the indexing model from the issue text.