code: def create_package(self, assay=None, mass=0.0, P=1.0, T=25.0,
                         normalise=True):
          
  
          if assay is None:
              return MaterialPackage(self, self.create_empty_assay(), P, T)
  
          if normalise:
              assay_total = self.get_assay_total(assay)
          else:
              assay_total = 1.0
  
          return MaterialPackage(self, mass * self.converted_assays[assay] /
                                 assay_total, P, T, self._isCoal(assay),
                                 self._get_HHV(assay))
docs: Create a MaterialPackage based on the specified parameters.
  
          :param assay:     Name of the assay to be used to create the package.
          :param mass:      Package mass. [kg]
          :param P:         Package pressure. [atm]
          :param T:         Package temperature. [°C]
          :param normalise: Indicates whether the assay must be normalised
            before creating the package.
  
          :returns: MaterialPackage object.