Finalize initial versioning

[gselzer]

We have (at least) two choices for the initial version of this project:

• 1.0.0, denoting the start of this new library
• 3.0.0, denoting the continued history from imglib/imglib2-ij

@ctrueden expressed a preference for the latter, while I like the former a bit better. @tpietzsch do you have a preference here?

[tpietzsch]

no preference

[ctrueden]

My pref is also very weak; I'm also fine with 1.0.0.