[Libreoffice] CONVERT_ADD() fixes

Hi,

Could you integrate this patch to fix some measurement conversion
factors of Calc?

http://www.numbertext.org/tmp/gallon.patch

Testing:

1 gallon is exactly 3.785411784 liter.
1 imperial gallon is exactly 4.54609 liter.

$ for i in 1 4 8 16 128 256 768 '(1/42)'; do echo "1/(3.785411784/$i)"
| bc -liq; done
1/(3.785411784/1)
.26417205235814841537 # 1 liter in gallons
1/(3.785411784/4)
1.05668820943259366151 # ~ in quarts
1/(3.785411784/8)
2.11337641886518732303 # ~ in pints
1/(3.785411784/16)
4.22675283773037464607 # ~ in cups
1/(3.785411784/128)
33.81402270184299716862 # ~ in ounces
1/(3.785411784/256)
67.62804540368599433725 # ~ in tablespoons
1/(3.785411784/768)
202.88413621105798301176 # ~ in teaspoons
1/(3.785411784/(1/42))
.00628981077043210512 # ~ in barrels

$ for i in 1 4; do echo "1/(4.54609/$i)" | bc -liq; done
1/(4.54609/1)
.21996924829908778752 # 1 liter in imperial gallons
1/(4.54609/4)
.87987699319635115010 # ~ in imperial quarts

Thanks in advance,
László