`unit` and `dimension` are inconsistent for `Level`s

[sostock]

For Levels (and mixed quantities based on Levels), unit discards the logarithmic part, dimension does not:

julia> unit(5u"dBV")      # NoUnits

julia> dimension(5u"dBV") # voltage
𝐋^2 𝐌 𝐈^-1 𝐓^-3

julia> unit(5u"dBV*m")      # meters (a length)
m

julia> dimension(5u"dBV*m") # voltage * length
𝐋^3 𝐌 𝐈^-1 𝐓^-3

I think the behavior of dimension makes sense. To fix unit, there are two possibilities:

1. Use the unit of the reflevel, i.e., unit(5u"dBV") == V
2. Make unit(::Level) throw an error

Edit: Related: #528.

[icweaver]

Sorry if this would be better as a new issue. I think I'm hitting this too over in JuliaAstro/DustExtinction.jl#58, specifically for mag support:

julia> using Unitful, UnitfulAstro

julia> x = 2u"mag"
2 mag

julia> unit(x)

Is there anything that can be done on our end?

Potentially related: #267, #265

[icweaver]

ahhh, TIL about logunit. I see this is what Plots.jl uses for their recipes, so nvm. Sorry for the noise!

[cgarling]

I would argue logunit should be documented if it's the main way to get the unit out of a logarithmic quantity. I opened #769 to add this documentation.