List all units and constants in the docs

[rafaqz]

Its not easy to find e.g. the Stefan Bolzmann constant. Searching the docs for Stefan returns nothing.

It would be nice if there was a page listing all the docs for all of these.

[ggggggggg]

Agreed, I just resorted to searching github to find the stephan boltzman constant, the boltzman constant and the charge of the electron. Maybe the constants could be re-exported into Unitful.Constants for easier tab completion as well?

[giordano]

Are you aware of PhysicalConstants.jl?

[ggggggggg]

Yes, but there are already lots of useful constants in Unitful, I prefer to use those and have them easily accessible when it has what I need already.

[rafaqz]

Totally, why use another package when we have the constants already.

[giordano]

Because I don't think constants should live here.

[giordano]

Especially since they are not well defined at all in this package: #81.

[rafaqz]

Thats a good point about Float64.

PhyisicalUnits.jl is not listed in the readme here, adding it would be a good step.

But still, will these constants be deleted here?