Multiple #@matrix parameters not supported

In a Matlab script started from ImageJ's script editor, a single `#@matrix` parameter works as expected and takes the current active image from ImageJ.

In analogy to `#@ Dataset`, I'd expect to get dropdown choice widgets for my parameters if I have more than one of them:

```
#@ matrix input1
#@ matrix input2
```

Instead, this show an error that a MatlabNumericArray is required but none is available.

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While [`MATLABNumericArrayPreprocessor`](https://github.com/imagej/imagej-matlab/blob/da50b03b539b0a5d608e63427ddf5ca1c9aa5027/src/main/java/net/imagej/matlab/MATLABNumericArrayPreprocessor.java) (`extends SingleInputPreprocessor`) deals with the former case, the latter currently doesn't work presumably because there is no direct conversion support for `ImagePlus` -> `MatlabNumericArray`.

We could add support for all the common image-type objects (e.g. `ImagePlus`, `Img`, `Dataset`) by chained conversion using a [delegate converter](https://github.com/scijava/scijava-common/blob/3ab383f20b69fe3a14d795215911025760ca1427/src/main/java/org/scijava/convert/AbstractDelegateConverter.java), but is this repository the right place to e.g. also contain the legacy `ij.ImagePlus <-> matlabcontrol.extensions.MatlabNumericArray`? @ctrueden?

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As an aside: the `#@matrix` script parameter alias is added in `scripting-matlab` by [`DefaultMATLABService`](https://github.com/scijava/scripting-matlab/blob/f9943e066c7aabb02f5376917c0e13fdac2b7adb/src/main/java/org/scijava/plugins/scripting/matlab/DefaultMATLABService.java#L88-L89)

Start with src/main/java/net/imagej/matlab/MATLABNumericArrayPreprocessor.java and compare its SingleInputPreprocessor behavior with the multiple-parameter case. Read the linked AbstractDelegateConverter and DefaultMATLABService references to determine where the conversion support belongs. Done means two #@ matrix parameters produce the expected dropdown choices and no missing MatlabNumericArray error.

Written by the indexing model from the issue text.