A Float Range Sequence

range only works with integers. Implement FloatRange(start, stop, step) behaving like a proper read-only sequence:

• len(r) is max(0, ceil((stop - start) / step)), computed with a tolerance so FloatRange(0, 1, 0.1) has length 10, not 11
• r[i] returns start + i * step (no accumulated error), supports negative indices and raises IndexError out of range; slices return a list
• iteration, reversed(), and x in r (true if x is within 1e-9 of an element)
• step of 0 raises ValueError; negative steps count down