Disallow StepRange{<:Quantity{<:AbstractFloat}}

[sostock]

For unitless numbers, StepRange enforces certain restrictions:

julia> StepRange(1.0, 2.0, 3.0)
ERROR: ArgumentError: StepRange should not be used with floating point
[...]

julia> StepRange(1, 1//2, 2)
ERROR: ArgumentError: StepRange{<:Integer} cannot have non-integer step
[...]

However, for unitful quantities, these are currently allowed:

julia> StepRange(1.0u"m", 2.0u"m", 3.0u"m")
(1.0:2.0:3.0) m

julia> StepRange(1u"m", (1//2)u"m", 2u"m")
(1//1:1//2:2//1) m

julia> ans[2]
ERROR: InexactError: Int64(3//2)
[...]

After this PR, the restrictions that apply to unitless numbers are also applied to unitful quantities. Closes #552.

Edit: We would have to drop support for Julia < 1.6 for this PR. I checked that it works with Julia 1.6 and 1.7.

[codecov-commenter]

Codecov Report

Merging #560 (e2e8fa9) into master (634071e) will decrease coverage by 0.41%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
- Coverage   87.93%   87.51%   -0.42%     
==========================================
  Files          16       16              
  Lines        1450     1450              
==========================================
- Hits         1275     1269       -6     
- Misses        175      181       +6     

Impacted Files	Coverage Δ
src/range.jl	90.10% <77.77%> (-1.46%)	⬇️
src/Unitful.jl	83.33% <0.00%> (-16.67%)	⬇️
src/quantities.jl	92.88% <0.00%> (-1.26%)	⬇️
src/units.jl	84.00% <0.00%> (-0.67%)	⬇️
src/user.jl	95.60% <0.00%> (+0.34%)	⬆️

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