bug#10554: 24.0.92; No units specified (dimensionless quantities in Emacs Calc)

[Jay Belanger]

> In Calc type
...
> Convert this to SI units with u c si RET
> - That's perfect. We get a dimensionless number.
...
> Now when we type again u c, Calc remembers what we did before.
> So as a unit it offers "default 1".

Calc should probably offer "si" as the default in this case.

> Let's take this default.
> So just type RET
>
> Error: No units specified
>
> In other words: if an expression with units simplifies to a
> dimensionless number, Calc does not accept the unit "1", though it
> even offers this unit as a default for calc-convert-units.

Calc shouldn't offer "1" as a default; I'll fix it so that it doesn't
do that anymore.

I'll also change it so that Calc can offer unit systems and composite
units as default units, but that should probably wait until after 24.1
comes out.

Jay