IO: restore broken API

Restored the openImg API that returns a single ImgPlus. Renamed the
List-based API to "openImgs".

TODO: same treatment for ImgOpener
TODO: should have refactored the openImg names. make sure nothing is
using deprecated methods.
ImgOpener: restore deprecated API

Renamed "List openImg" methods to "openImgs". Restored the "openImg" API
which returns a single ImgPlus and marked it deprecated.

All openImg implementations are trivial delegates to the corresponding
openImgs method, with a get(0) on the result.
SCIFIOConfig: restore removed API

Restored the imgOenerSetIndex(int) method to limit API breakage.