physics: optimizations, created a lenght() function

This should make the code more readable. You might notice how we fled away from long double type for the lenght. And with good reason as well, we don't need a billion digit precision here, there's no way two objects will be so damn close to make that matter. I think I should also scale the rest of the units accordingly, due to the fact floating point operations are faster than double or long double... the latter of which being highly platform/compiler specific and inprocessable by the FPU.