Updated the CUDA MHD solver. Main updates include:
1. Changed CUDA MHD grid member files from .cu to .C. This makes the CUDA files more compatible when using non-gnu compilers. All the CUDA kernel are now centralized in CudaMHD.cu.
2. All the function interfaces in Grid_CudaMHD.C are rewritten to get rid of the long argument list.
3. CUDA documentation in usr_guide updated.
4. InitCUDA call in enzo.C updated with #ifdef guide and InitCUDA itself also updated.