Unitful as a package extension

[JeffFessler]

No description provided.

[JeffFessler]

On 2nd thought, let's keep it Number to support other number types besides Unitful

Here is how I might have tried to do it with extensions, but it may not work anyway:

    _ext = Base.get_extension(@__MODULE__, :MIRTjim_Units)
    if isnothing(_ext)
        const RealU = Real
    else
        const RealU = Union{Real, _ext.Unitful.Length}
    end