ImageJ2Options: add getter

Added a public isUseSCIFIO accessor.
DefaultLegacyOpener: simplify isUseSCIFIO logic

Updated to use the API recommended by Curtis Rueden.
WelcomeToImageJ2: correct author

Copied from a service created by Curtis Rueden!
Remove unnecessary persist=false

Service @Parameters do not need to be labeled persist = false in
OptionsPlugins.