Support multiple GPUs

Hello @frauzufall , hello future-self,

in clij-core, in methods of the `Kernel` class, there is always a parameter `CLIJ clij`. Idea behind was potentially supporting multiple GPUs. This doesn't work in the current prototype of clij-ops, because all generated Ops get their CLIJ instance via `CLIJ.getInstance()`. Spontaneously I don't know how to fix this, but I think there is a way...

Thanks for listening ;-)

Cheers,
Robert

Start by reading the Kernel class methods in clij-core and how generated Ops in clij-ops obtain their CLIJ instance via CLIJ.getInstance(). Trace how the CLIJ parameter is intended to flow through these entry points. Done means generated Ops can use the appropriate CLIJ instance so multiple GPUs are supported.

Written by the indexing model from the issue text.