Set license to GPL v3, not BSD

We depend on the SLIM Curve library, which is GPL v3. So by the terms of
the GPL, this library must also be licensed under the GPL v3 or later.
Tweak project name

We'll refer to it as the "Spectral Lifetime Analysis" plugins for
ImageJ, and deprecate the "SLIM Plugin" title, which was redundant
and inconsistent with all the other ImageJ plugins out there.