KDBush worker fails due to missing KDBush

[flekschas]

As initially identified by openjournals/joss-reviews#7059 (comment), the KDBush worker introduced in #178 does not work due to:

ReferenceError: KDBush is not defined

[flekschas]

It turns out that for some reason, the class is called KDBush2 in the worker instead of KDBush