def to_base_unit
      unit = @unit.base_unit
      val  = unit.convert(self)
      expr = unit.unit_string
      self.class.new( val, expr, unit )
    end
def to_base_unit
      unit = @unit.base_unit
      val  = unit.convert(self)
      expr = unit.unit_string
      self.class.new( val, expr, unit )
    end
Conversion to base unit.
 Returns the quantity converted to a base unit.
 @return [Phys::Quantity] a quantity in the base unit.
 @raise  [Phys::UnitError] if unit conversion is failed.