ImgOpener: allow opening of all images

ImgOpener now returns a list of ImgPlus objects, and can be instructed
to open specific ranges or all known images in a dataset through
configuration with the SCIFIOConfig class.

This required updates to the SCIFIOCellImg so that cells are now
associated with a particular index, to ensure the proper data is opened
when requested.

Added unit tests using .fakes to ensure the new functionality is working
as intended. This is a breaking API change.
Rename SubRegion -> ImageRegion

SubRegion is a confusing name that doesn't really convey the intent of
this class. The intended use is a region of an image, thus the rename.
Rename DimRange -> Range

As this class is now being used for image indices, it is no longer just
a "dimensional" range of values, but something more general.

Updated the javadoc to properly reflect this as well.