These functions allow you to spin for a short time using only the CPU.
You should always prefer Software Timers (see modm:processing:timer) over these
blocking delay functions. However, when modm::Clock is not set up yet, or
when you need very small delays (for example to bit-bang a protocol), you need
to use these delay functions.
The only guarantee given to you is to delay for at least the specified time.
Note that invocation of interrupts during spinning may add delay too.
No real-time guarantees!
You must not rely on delay functions for ANY time-keeping!
Delay functions work at any CPU clock speed, even if changed dynamically and
are available very early in the startup process at hardware-init time.
Use the largest time unit possible!
Correct behavior is not guaranteed for delays over 1000ns, us or ms!