Consider removing C headers and replacing with Kokkos math functions, etc.

[felker]

Leftover from #582. Besides the necessary omp.h, mpi.h, hip/hip_runtime.h, unistd.h, and sys/stat.h, I see the following C deprecated (since C++11) .h headers imported in src/:

• math.h
• float.h
• stdio.h

I haven't searched comprehensively, but so far I havent found a case where the deprecated C headers are necessary or of missing coverage by a suitable Kokkos math function
fabs. Generally you dont want to pollute global namespace, so even replacing stdio.h with cstdio would be preferable (vs. the even better Kokkos::printf or <iostream>).

Should double check that there are no GCC and/or CUDA compatibility issues, as raised in #582 (comment)