populate combined group select lists with units from other groups

[honzi]

Some unit groups are combinations of units from other unit groups, such as the speed group being a combination of length over time. These combined groups should not have their own formulas, but rather automatically use all units in both of the other groups to populate their select lists.

• density is mass over volume
• fuel is length over volume, sometimes reversed
• mass flow is mass over time
• speed is length over time
• volumetric flow is volume over time