Use sensible Fortran flags

At the moment the fortran code is compiled with minimal flags, and therefore the compiler has its hands tied. However, the code is written in a way which should be easily vectorisable by the compiler.

Ekatherine from RSC is currently testing: -xCORE-AVX512 -fma -align array64byte

and has mentioned that -qopt-prefetch=3 worked well with the prototype code.

To be updated...