GPU support

[pmocz]

We should explore porting MESA to GPUs. Here is one approach for offloading some of our most intensive calculations: the block tridiagonal matrix solve

cuSparse has a tridiagonal solver we could use: https://docs.nvidia.com/cuda/cusparse/index.html?highlight=tridiagonal#tridiagonal-solve

For interfacing Fortran with GPUs, we can use CUDA Fortran to call directly. Here’s the documentation for doing that with cuSPARSE: https://docs.nvidia.com/cuda/cusparse/#cusparse-fortran-bindings