Additive identity requires a unit

[haykam821]

Despite zero being the additive identity in the following expression, a unit conversion error is thrown (in version 1.5.2):

> 1 meter + 0
Error: cannot convert from unitless to meters: units 'unitless' and 'meter' are incompatible

However, explicitly specifying the unit results in the correct output:

> 1 meter + 0 meters
1 meter

I would expect the unitless version to work given that multiplying or dividing a value by a unitless 1 works fine.

[printfn]

Thanks for the report, I agree it would make sense to support this.

[printfn]

I've implemented this now; I'll do a bit more testing to make sure there's no unintended breakage but I think it should be good.