Units plugin doesn't handle unit expressions

[Jonathan Hitchcock]

The units plugin has an advanced parser that can parse and interpret compound unit expressions. The regex in the units plugin only handles the simplest of cases:

12:18 <Vhata> Comfrey: convert cm^3 to gallons
12:18 <Comfrey> Vhata: cm^3 = 0.00026417205 gallons

12:18 <Vhata> Comfrey: convert arabicfoot * arabictradepound * force to ft lbf
12:18 <Comfrey> Vhata: What?

12:19 <Vhata> Comfrey: convert furlongs per fortnight to m/s
12:19 <Comfrey> Vhata: Sorry...

12:03 <tumbleweed> Comfrey: convert 30 miles to km/h
12:03 <Comfrey> tumbleweed: What?

Furthermore, by separating "value" out in the regex, it enforces the idea that a unit expression can be, at its most complex, "[n] units", whereas in reality, the quantity is an interal part of the expression:

11:58 <Vhata> Comfrey: convert 3 km to miles
11:58 <Comfrey> Vhata: 3 km = 1.8641136 miles

12:01 <Vhata> Comfrey: convert 2 ft 3 ft 12 ft to stere
12:01 <Comfrey> Vhata: Sorry...

I rather think that we should let GNU units decide what is/is not a valid expression, instead of trying to parse it ourselves.