we had both a class member bandjoin, and an instance member
Vips.Image.bandjoin([i1, i2, i3..])
i1.ibandjoin([i2, i3..])
this was confusing and annoying ... get rid of the class one and just
use bandjoin everywhere, so this is now the way to do it:
i1.bandjoin([i2, i3..])
oh argh class and instance methods are in the same namespace, so we have
to rename the instance one as ibandjoin
also, start adding a test for arrayjoin
