Wrong output of convert.clip ops

As discussed [on gitter](https://gitter.im/imagej/imagej-ops?at=5aa671c0e4ff28713a21aa01), the `convert.clip` op returns wrong results when used like e.g. in this Groovy script:

```groovy
#@ OpService ops

import net.imglib2.type.numeric.integer.UnsignedByteType
import net.imglib2.type.numeric.real.FloatType

clipOp = ops.op("convert.clip",new UnsignedByteType(), new FloatType())

println new FloatType(257) // 257.0
println new UnsignedByteType(257) // 1
println ops.run(clipOp, new UnsignedByteType(), new FloatType(257)) // 0 !
```

---

@ctrueden has started work on fixing the API of the convert ops here:
https://github.com/imagej/imagej-ops/compare/convert-api

Start with the Groovy reproduction in the issue and compare the expected values with the result from ops.run(clipOp, ...). Review the convert API changes in the linked convert-api comparison; done means convert.clip returns the expected clipped value for the shown UnsignedByteType and FloatType inputs.

Written by the indexing model from the issue text.