[Insight-developers] IO Factories

[Bill Hoffman]

I have checked in the following:

1. I added a new pure virutal to ImageIOBase:
void ImageIOBase::ReadImageInformation() = 0 ;

2. I removed ReadHeader from PNGImageIO and added ReadImageInformation, and
made CanReadFile only check if the file is a valid png image file.

3. I changed MetaImageIO and itkVOLImageIO to implement ReadImageInformation, for
now I just called ReadHeader for both of these classes, but the classes should be
fixed.

-Bill