Add support for summing multiple images into a single JP2

[Keith Hughitt]

In order to improve the signal-to-noise ratio for some types of images (e.g. AIA 94), it may be useful to sum several images together and generate a single JPEG 2000 image.

The XML header for such an image could include all three original headers, along with some information about the summing process itself, or some other metadata that the clients can use, e.g.:

<meta>
    <summary>
    </summary>
    <fits1>...</fits3>
    <fits2>...</fits2>
    etc.
</meta>

Since Helioviewer currently only uses every third image, this could be an easy way to improve the image quality without increasing the storage or bandwidth requirements.