Add gpu embedded backend #28

Add additional embedded backend running on the GPU
Potential packages to use: https://github.com/JuliaGPU/CUDA.jl
Many operations (e.g. arithmetic between fields) might directly work as soon as the broadcast is GPU aware, others e.g. remap need to custom handling. For the broadcast this is the central loop: https://github.com/GridTools/GridTools.jl/blob/main/src/embedded/cust_broadcast.jl#L260