Implement clamp for f64 Quantities

[Kyle Carow]

Recently tried to use clamp on an f64 quantity, but ran into this issue:

Quantity::clamp is only implemented via the Ord trait:
impl Ord for Quantity

f64 and f32 implement clamp manually (which makes sense, as they aren't Ord - though I'm unsure why clamp couldn't have been implemented in PartialOrd given that it only uses PartialOrd comparisons, oh well)
f64::clamp
f32::clamp

Could clamp (and maybe similar methods) be implemented for f64 Quantities? This would be used widely throughout my code, where I otherwise have doubled-up min/max calls.