Fortran standard sleep_ms()

OS/compiler-independent "sleep" Fortran subroutine

The simplest robust way to implement Fortran "sleep_ms(milliseconds)" in a standard way is using C++ as in sleep.cpp and sleep_std.f90

Two other methods are shown. POSIX nanosleep in sleep.c takes noticeably more code than C++. Simply binding to C functions is less robust in reference/.