Fix image links in api test projects (ilastik#2859)

projects could not be opened - images have been renamed (I guess). Made
sure data is stored with relative links.

Also removed (hopefully the last) occurrence of lena in our test-data.

Details for zip contents:

* `2464_PixelClassification_xy_input.ilp` - unchanged
* `Data_1channel.png` - unchaged
* `Data_3D.npy` - unchanged
* `Data_3channel.png` - replaced lena with scaled down astronaut (and removed one row of pixels to make it non-square)
* `PixelClass.ilp` - restored relative link to `Data_1Channel.png`
*  `PixelClass3Channel.ilp` - recreated with new data
* `PixelClass3D.ilp` - restored relative link to `Data_3D.npy`
* `PixelClass3D_2D_3D_feature_mix.ilp` - restored relative link to `Data_3D.npy`
* `PixelClassNoClassifier.ilp` - restored relative link to `Data_1Channel.png`
* `PixelClassNoData.ilp` not modified