use scale factor that is passed via the SCIFIOConfig
Check if the currently read file really contains images and throw a knip-exception, if not.
Wrap the reader used in the ScifioImgSource with a UnclosableReaderFilter to prevent him from being closed.

The reason: if an image is opened via the ImgOpener the metadata is reset afterwards (even if just one series of the current image file is read). To read another series of the same file the file has to be parsed again, what might be a bit expensive for some file formats.
Now, as the reader is unclosable, it is closed not before all desired series have been read.
PdfFormat can either read now the whole pdf document as images (page-wise) or extract the images within the document.
Everytime assume a channel dimension for now.
PDF-document specific options added to the image reader:
- dialog components
- settings models
- updated node description
Merge branch 'readpdfs' of https://github.com/knime-ip/knip into readpdfs

Conflicts:
	org.knime.knip.io/src/org/knime/knip/io/formats/PdfFormat.java