add metrix prefixes and 'b' shorthand for barns

picobarn = pb is a very common unit in High Energy Physics

numbat/modules/units/misc.nbt

@@ -9,7 +9,8 @@ unit bar: Pressure = 100 kPa
 @aliases(angstroms, Å: short)
 unit angstrom: Length = 1e-10 meter
 
-@aliases(barns)
+@metric_prefixes
+@aliases(barns, b: short)
 unit barn: Area = 1e-28 meter^2
 
 @metric_prefixes