Fix logarithmic units

[gustaphe]

Description

Closes #4700

Partially based off of work by @ykonter (who is not in zenodo, as far as I can tell). Creates underscored extensions of Unitful's unit and ustrip to deal with logarithmic units.

This is not perfect, note how the y axis in the example doesn't rescale on the plot! call. But there is more than one interpretation of what that call should do anyway, so perhaps good enough should be left good enough here.

Example

julia> plot((1:10)u"dBm", (1:10)u"dB"; seriestype=:scatter)

julia> plot!((1:10)u"mW", (1:10); seriestype=:scatter)

Attribution

• I am listed in .zenodo.json (see How to get acknowledged #3503)

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.40 🎉

Comparison is base (4400dba) 89.99% compared to head (988213f) 90.40%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4707      +/-   ##
==========================================
+ Coverage   89.99%   90.40%   +0.40%     
==========================================
  Files          40       40              
  Lines        8359     8725     +366     
==========================================
+ Hits         7523     7888     +365     
- Misses        836      837       +1     

see 33 files with indirect coverage changes

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[t-bltg]

Xref jw3126/UnitfulRecipes.jl#72 and jw3126/UnitfulRecipes.jl#73 (to my current knowledge).
I think a partial fix is better than nothing.

[gustaphe]

No documentation changes. The only thing I think is not in line with previous documentation is the conversion weirdness around dimensionless units vs logarithmic ones, and I'm not sure Plots.jl is the place you should be reading about that.