Clean up image description population
Add 'isFiji()' method and remove 'IS_FIJI' constant

This prevents the Fiji-ness from being detected incorrectly if the
Updater class is loaded before the ImageJ instance is created.
Cellworx: fix off-by-one if all pixels files are missing

This won't allow .htd files without TIFFs to import, but it makes the
exception that is thrown more informative.

Fixes #6332.
merge develop -D org -I include -E exclude: PR 528 (Tile performance improvements for VMS and NDPI)
merge develop -D org -I include -E exclude: PR 767 (Fix Channel.Color interpretation in ImageJ)
merge develop -D org -I include -E exclude: PR 775 (Add JWlz to Ivy dependencies)
merge develop -D org -I include -E exclude: PR 788 (Do not prompt to update to the latest stable version if Fiji is used)
merge develop -D org -I include -E exclude: PR 790 (Update netCDF to 4.3.19)
merge develop -D org -I include -E exclude: PR 791 (Add support for PCORAW data)
merge develop -D org -I include -E exclude: PR 796 (Zip: improve reader and handle performance)
merge develop -D org -I include -E exclude: PR 797 (Zeiss CZI: fix image reading for linescan images)
merge develop -D org -I include -E exclude: PR 800 (Cellworx: fix off-by-one if all pixels files are missing)