Make phi0 a DerivedUnit (#57)

Author: NoureldinYosri

tunits/core/cython/derived_unit_data.pyx

@@ -78,6 +78,7 @@ __SI_DERIVED_UNITS = [
     DerivedUnitData('lx', 'lux', 'lm/m^2', use_prefixes=True),
     DerivedUnitData('Bq', 'becquerel', 'Hz', use_prefixes=True),
     DerivedUnitData('l', 'liter', 'm^3', exp10=-3, use_prefixes=True),
+    DerivedUnitData('phi0', 'magnetic_flux_quantum', 'J*s/C', value=2.067833831170082e-15, use_prefixes=True)
 ]
 
 __OTHER_UNITS = [

tunits/units.py

@@ -953,7 +953,7 @@
 petawatt = core.default_unit_database.known_units['petawatt']
 petaweber = core.default_unit_database.known_units['petaweber']
 pg = core.default_unit_database.known_units['pg']
-phi0 = hplanck / (2 * e)
+phi0 = core.default_unit_database.known_units['phi0']
 picoampere = core.default_unit_database.known_units['picoampere']
 picobecquerel = core.default_unit_database.known_units['picobecquerel']
 picocandela = core.default_unit_database.known_units['picocandela']

tunits/units_with_dimension.py

@@ -953,7 +953,7 @@
 petawatt = core.Power(core.default_unit_database.known_units['petawatt'])
 petaweber = core.MagneticFlux(core.default_unit_database.known_units['petaweber'])
 pg = core.Mass(core.default_unit_database.known_units['pg'])
-phi0 = core.MagneticFlux(hplanck / (2 * e))
+phi0 = core.MagneticFlux(core.default_unit_database.known_units['phi0'])
 picoampere = core.ElectricCurrent(core.default_unit_database.known_units['picoampere'])
 picobecquerel = core.Frequency(core.default_unit_database.known_units['picobecquerel'])
 picocandela = core.LuminousIntensity(core.default_unit_database.known_units['picocandela'])