Custom units

[Anonymous]

It would be great to have Custom Units definition. For example, "mason's arshin" is equal to 0.76 m. Entering the name and value in the preferences could offer an easy way to use such a measurments. 

[klm__]

I'd personally like the option to use smaller units, such as microns or thou, for sketches of particularly small parts.

[Toby.Roworth]

I think it's important that can be defined using expressions, not just literals/constants. (micron = mm / 1000)