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.

[minnerbe]

I finally tested this on real data. The overhead of doing on-the-fly correction to just loading and transforming a single dataset is about 20-25%, which seems reasonable to me.

@mkitti please have a look and let me know if you run into any problems. There are a couple of xml files that show how to specify flatfield images and coding agents should be pretty good at extending those to cloud-hosted data. This was only tested for two setups (one local, one s3-hosted), so I still expect some rough edges.

[mkitti]

Also pinging @indecisiveuser for testing.