Constructor/conversion for dimensionless quantities

[sostock]

For pretty much all types T <: Number, calling T(x) and convert(T, x) are equivalent. However, for dimensionless unitful quantities, this is not the case:

julia> Float64(5u"mm/m")
5.0 mm m^-1

julia> convert(Float64, 5u"mm/m")
0.005

I think this is confusing and should at least be documented.

I also think that it’s very unfortunate that the Float64 constructor does not actually return a value of type Float64. I would rather have a dedicated function for converting the numerical value of a quantity to a different type. Of course, this can only be changed in 2.0.