Better test suite

[ahnlabb]

• [ ] Test with open formats using Images.jl
• [ ] Test with small sample images of different proprietary formats. (Maybe some sample images from the loci website could be used: sample data.)
• [ ] Use Bio-Formats to write a known test image to different formats (either through julia or separate tool). Might not add anything useful since Bio-Formats can only export to open formats.