Add support for running examples on GPU and test on GPU

[mtfishman]

Add flexibility to run examples on GPU similar to ITensor/ITensorTDVP.jl#22, and test running on GPU similar to how it is done in NDTensors.jl and ITensors.jl. We can test running on GPU backends using JLArrays as discussed in #39.

[mtfishman]

See also ITensor/ITensors.jl#1553 for a workflow for running DMRG tests on various GPU backends.