GPU stuff

[gdalle]

• Set up CI with NVIDIA server
• Sparse matrix types
• Kernel fusion
  • CUDA's native API (mathdx)
  • Reactant integration

[klamike]

Reactant actually has a KA backend, IIRC. Although when I tried it a while ago it wasn't super straightforward. Worth taking a look again.

[gdalle]

GPU CI set up in #39