add getCache() to ViewerImgLoader interface

src/main/java/bdv/BigDataViewer.java

src/main/java/bdv/ViewerImgLoader.java

src/main/java/bdv/img/catmaid/CatmaidImageLoader.java

src/main/java/bdv/img/hdf5/Hdf5ImageLoader.java

src/main/java/bdv/img/remote/RemoteImageLoader.java