Change package name from ITensorsGPU to ITensorGPU

[mtfishman]

It would be nice to have a consistent naming convention for packages in the ITensor ecosystem, and I was thinking the convention should be that they are named ITensor*.jl (instead of ITensors*.jl) since I think it reads better. ITensors.jl really only uses an s since the module name can't be the same as the type ITensor.

We could follow these guidelines to change the name. I also plan to do the same with other packages like ITensorsVisualization.jl -> ITensorVisualization.jl and ITensorsGrad.jl -> ITensorGrad.jl.