Minor addition to units/constants

modules/physics/constants.nbt

@@ -63,4 +63,7 @@ let planck_time: Time = sqrt(ℏ gravitational_constant / speed_of_light^5) -> s
 let planck_temperature: Temperature = sqrt(ℏ speed_of_light^5 / (gravitational_constant * k_B^2)) -> K
 
 # Planck energy
-let planck_energy: Energy = sqrt(ℏ speed_of_light^5 / gravitational_constant)
+let planck_energy: Energy = sqrt(ℏ speed_of_light^5 / gravitational_constant) -> J
+
+# Bohr radius
+let a0: Length = 4 pi ε0 ℏ^2 / (electron_charge^2 electron_mass)

modules/units/us_customary.nbt

@@ -3,7 +3,7 @@ use units::imperial
 
 # US liquid gallon
 @aliases(gallons, gal: short)
-unit gallon: Volume = 0.003785411784 meter^3
+unit gallon: Volume = 3.785_411_784 L
 assert_eq(1 gallon, 231 inch^3, 0.001 inch^3)
 
 # US liquid pint