Make "Scale" command optionally multi-threaded

[acardona]

With large datasets nowadays, often an ImageJ stack is virtual, and lacking a cache, often takes too much time to scroll back and forth across Z. A simple solution is to duplicate a range of sections. But since these are virtual, fetching the files, scaling, interpolating, etc. takes time, so the duplication takes long. Many of these operations could run entirely in parallel, hence, a simple solution is to add a checkbox "multithreaded" to the Scaler.java plugin GenericDialog, with the corresponding flag for the macro recorder, to duplicate the stack using one Thread per slice to duplicate, for as many threads as specified in the ImageJ options.