Fix im3 metadata

I neglected to call MetadataTools.populatePixels in the IM3Reader's initFile method. That was preventing the OME-XML generator from populating the image planes.

In addition, I added the IM3Reader to readers.txt

Thanks in advance for looking at this.
Could you please use ```makeFilterMetadata()``` instead of ```getMetadataStore()```?  That wraps the result of ```getMetadataStore()``` such that the return value of ```isMetadataFiltered()``` will always be respected.