using logarithmic quantities

[m-wells]

(I posted this in an old pull request #103 and realized nobody would ever see it there)

I've been struggling with this all day and finally got it to work. I had to do a lot of digging and I think the documentation could benefit from some clarification. I deal with logarithmic quantities all the time in astronomy so this is quite useful to me.

julia> using Unitful

julia> import Unitful:d

julia> @logscale Log₁₀ "Log₁₀" LogBase10 10 1 false
Log₁₀

julia> @logunit log₁₀d "log₁₀(days)" LogBase10 1d
log₁₀(days)

julia> Unitful.isrootpower_dim(::typeof(dimension(d))) = false

julia> foo = 3*log₁₀d
3.0 log₁₀(days)

julia> uconvert(u"d", foo)
1000.0 d

julia> bar = 0.3*log₁₀d
0.3 log₁₀(days)

julia> uconvert(u"d", bar)
1.9952623149688795 d

The isrootpower_dim is required to avoid the following:

julia> foo = 3*log₁₀d
ERROR: undefined behavior. Please file an issue with the code needed to reproduce.
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] isrootpower_dim(::Unitful.Dimensions{(Unitful.Dimension{:Time}(1//1),)}) at /home/mark/.julia/packages/Unitful/W0mMi/src/logarithm.jl:200
 [3] isrootpower(::Quantity{Int64,𝐓,Unitful.FreeUnits{(d,),𝐓,nothing}}) at /home/mark/.julia/packages/Unitful/W0mMi/src/logarithm.jl:199
 [4] fromlog(::Type, ::Quantity{Int64,𝐓,Unitful.FreeUnits{(d,),𝐓,nothing}}, ::Int64) at /home/mark/.julia/packages/Unitful/W0mMi/src/logarithm.jl:116
 [5] *(::Int64, ::Unitful.MixedUnits{Level{Unitful.LogInfo{:LogBase10,10,1},1 d,T} where T<:Number,Unitful.FreeUnits{(),NoDims,nothing}}) at /home/mark/.julia/packages/Unitful/W0mMi/src/logarithm.jl:147
 [6] top-level scope at none:0