Fix type promotion

[hyrodium]

This PR fixes #86.

julia> using Unitful: m, s

julia> using IntervalSets

julia> x = 1m .. 5m
1 m..5 m

julia> y = 1m .. 2s
ERROR: promotion of types Unitful.Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}} and Unitful.Quantity{Int64, 𝐓, Unitful.FreeUnits{(s,), 𝐓, nothing}} failed to change any arguments
Stacktrace:
 [1] error(::String, ::String, ::String)
   @ Base ./error.jl:42
 [2] sametype_error(input::Tuple{Unitful.Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}, Unitful.Quantity{Int64, 𝐓, Unitful.FreeUnits{(s,), 𝐓, nothing}}})
   @ Base ./promotion.jl:374
 [3] not_sametype(x::Tuple{Unitful.Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}, Unitful.Quantity{Int64, 𝐓, Unitful.FreeUnits{(s,), 𝐓, nothing}}}, y::Tuple{Unitful.Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}, Unitful.Quantity{Int64, 𝐓, Unitful.FreeUnits{(s,), 𝐓, nothing}}})
   @ Base ./promotion.jl:368
 [4] promote
   @ ./promotion.jl:351 [inlined]
 [5] (ClosedInterval)(left::Unitful.Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}, right::Unitful.Quantity{Int64, 𝐓, Unitful.FreeUnits{(s,), 𝐓, nothing}})
   @ IntervalSets ~/.julia/dev/IntervalSets/src/interval.jl:30
 [6] ..(x::Unitful.Quantity{Int64, 𝐋, Unitful.FreeUnits{(m,), 𝐋, nothing}}, y::Unitful.Quantity{Int64, 𝐓, Unitful.FreeUnits{(s,), 𝐓, nothing}})
   @ IntervalSets ~/.julia/dev/IntervalSets/src/interval.jl:85
 [7] top-level scope
   @ REPL[4]:1

Note that we now have a new issue around missing. I'll open it soon.

[JeffFessler]

Now that #94 and #98 seem done, can you rerun the CI for this PR to see if it works OK?
Or does something else need to be done first?

[codecov]

Codecov Report

Merging #93 (34ed8a4) into master (02d641f) will decrease coverage by 0.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
- Coverage   99.26%   98.88%   -0.39%     
==========================================
  Files           3        3              
  Lines         272      268       -4     
==========================================
- Hits          270      265       -5     
- Misses          2        3       +1     

Impacted Files	Coverage Δ
src/interval.jl	99.13% <100.00%> (-0.03%)	⬇️
src/IntervalSets.jl	98.40% <0.00%> (-0.80%)	⬇️

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[hyrodium]

I think this is ready to merge. I'll wait for some approvals.