`StepRange` with float-based quantities should error

When dealing with floating-point numbers, `StepRangeLen` should be used instead of `StepRange`. However, this is currently allowed:
```julia
julia> StepRange(1.0u"m", 1.0u"m", 3.0u"m")
(1.0:1.0:3.0) m
```

I would argue that it should error, like it does for unitless numbers:
```julia
julia> StepRange(1.0, 1.0, 3.0)
ERROR: ArgumentError: StepRange should not be used with floating point
[...]
```