Add Qiskit GPU simulator

[ohuettenhofer]

This pull request depends on #368.

I added support for running the Qiskit simulator on the GPU by creating a subclass that changes the device name. I used the same structure as for the Qulacs GPU simulator.

Using this requires the qiskit-aer-gpu package.

I also commented out the package check for qulacs-gpu, because that package is not maintained, instead you have to compile Qulacs with GPU support, see qulacs/qulacs#623 (comment).

[kottmanj]

Great - Thanks a lot!
does it run on CPUs as well? (asking for the github CI runner)

[ohuettenhofer]

does it run on CPUs as well? (asking for the github CI runner)

I don't know, but I don't think so.
But I didn't properly catch the errors when the GPU packages aren't installed, that's why the runners were failing. It should be fixed now (but it just skips the GPU backends).