Add support for `net.imagej.roi.DefultROITree` conversion to Python

@elevans is already working on this.

Since Apr 21, 2023.

If you have a `net.imagej.Dataset` with an attached `net.imagej.roi.DefaultROITree` and ask for an `xarray.DataArray` via `ij.py.to_xarray()` the conversion will fail with:

```python
...
File "/home/edward/Documents/repos/loci/scyjava/src/scyjava/_convert.py", line 82, in _raise_type_exception
raise TypeError("Unsupported type: " + str(type(obj)))
TypeError: Unsupported type:
```
This happens because at some point during the `Dataset` to `DataArray` step we look through the dataset's properties, which contains the `DefaultROITree`. This `DefaultROITree` is passed on to `scyjava` where it then fails to convert.

Here is a minimal example using the test data in the PyimageJ repo.

```python
import imagej
import scyjava as sj

# init pyimagej
ij = imagej.init(mode='interactive')
print(f"ImageJ Version: {ij.getVersion()}")

# get ImageJ resources
OvalRoi = sj.jimport('ij.gui.OvalRoi')
ov = sj.jimport('ij.gui.Overlay')()

# load data
imp = ij.IJ.openImage('/doc/sample-data/test_timeseries.tif')

# draw ROI
roi = OvalRoi(56, 79, 102, 100)
imp.setRoi(roi)
ov.add(roi)
imp.setOverlay(ov)
imp.show()

# convert imp to dataset
ds = ij.py.to_dataset(imp)

# this call fails
# ij.ui().show(ds)
```