Make minimum/maximum generic for Frequencies

[platawiec]

Before:

julia> import Unitful: mm
julia> using FFTW
julia> fftfreq(10, 1.0mm);
julia> minimum(f)
ERROR: DimensionError: 0.0 and 0.1 mm are not dimensionally compatible.

After:

julia> import Unitful: mm
julia> using FFTW
julia> fftfreq(10, 1.0mm);
julia> minimum(f)
-0.5mm

I didn't include a test, but if you are ok w/ including Unitful in test dependencies I can put that in.

[codecov]

Codecov Report

Merging #54 (7794eaa) into master (193d035) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #54   +/-   ##
=======================================
  Coverage   52.63%   52.63%           
=======================================
  Files           2        2           
  Lines          95       95           
=======================================
  Hits           50       50           
  Misses         45       45           

Impacted Files	Coverage Δ
src/definitions.jl	52.12% <100.00%> (ø)

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[timholy]

This is great. Just needs a test, and it would be fine to add a test dependency.

[stevengj]

I guess there's still no way to use https://github.com/JuliaLang/julia/blob/master/test/testhelpers/Furlongs.jl? (JuliaLang/julia#30692)

[timholy]

Presumably one can manually load testhelpers.jl via Sys.BINDIR, but it's a little ugly.

[stevengj]

We can use Unitful.jl as a test dependency for now. If JuliaLang/julia#39852 is merged we can switch to that in a future release.

[timholy]

Thanks @platawiec!

JuliaRegistries/General#31060