bug#12687: 24.2.50; calc volume units conversion

[Jay Belanger]

> emacs -Q
> C-x **    ;start calc
> 'gal RET  ;enter 1 gallon
> uc        ;try to convert to new units
>
> At this point calc should prompt for new units, but it does not.

Oh; thanks for catching this!

> A workaround for me is to redefine the math-get-standard-units
> function:

That works for this case (but for the wrong reasons ... for example, if
you convert gallons to liters and then convert gallons again, it won't
offer liters as a default).  The problem was that `math-extract-units'
wasn't dealing with powers of units (and the standard units for volume
are m^3).  This has been fixed in bzr.

Thanks again,
Jay