Offer to convert types if needed when saving

[ctrueden]

Some image formats can only save to certain image types. When attempting to save an image which is not one of those types, an exception similar to the following occurs:

[ERROR] net.imglib2.io.ImgIOException: Pixel type not supported. Please convert your image to a supported type.
    at net.imglib2.io.ImgIOUtils.makeType(ImgIOUtils.java:227)
    at net.imglib2.io.ImgSaver.populateMeta(ImgSaver.java:631)
    at net.imglib2.io.ImgSaver.initializeWriter(ImgSaver.java:596)
    at net.imglib2.io.ImgSaver.saveImg(ImgSaver.java:184)
    at imagej.core.commands.io.SaveAsImage.run(SaveAsImage.java:141)

We should at minimum make this message nicer (display a dialog instead of an exception dump). But really, it would be nice to offer to convert to a supported output format rather than just failing. We might need to add some API to SCIFIO for this.

Migrated from Fiji bug #315 and OME ticket #5323. See also ImageJ Trac ticket #1704.