Allow timer frequencies below 1 Hz

[TheZoq2]

A common complaint I hear about the crate, and have myself, is that timers which run for longer than 1 second.

One issue is that it's kind of hard to (without panics) check if a timer value is feasible. One possibilty might be to allow lower level access to the timer registers to set your own pre-scalers etc. if the variants provided by the HAL aren't satisfactory.

Similar to #126 I suppose

[David-OConnor]

See this issue on F3. The set_period() function I posted there handles this. It should work similarly on f1.