Modernize lazy flatfield correction code

[minnerbe]

This PR aims to update the lazy flatfield correction code in net.preibisch.mvrecon.fiji.spimdata.imgloaders.flatfield in several ways:

• add an image loader that implements the ViewerImgLoader interface
• use improved downsampling code for on-the-fly downsampling of flatfields
• use the N5 api to load the flatfields to also be able to work with chunked / cloud storage

This is still a draft as more features will likely still be added.