def load
      if describe && !loaded?
        @@dimensions << self
      elsif describe
        raise Exceptions::InvalidDimensionError, "A dimension instance with the same physical quantity already exists"
      else
        raise Exceptions::InvalidDimensionError, "Cannot load dimensions without physical quantity description"
      end
    end
def load
      if describe && !loaded?
        @@dimensions << self
      elsif describe
        raise Exceptions::InvalidDimensionError, "A dimension instance with the same physical quantity already exists"
      else
        raise Exceptions::InvalidDimensionError, "Cannot load dimensions without physical quantity description"
      end
    end
Load an already instantiated Dimensions object into the @@dimensions class
 array, from which it will be accessible as a universal representation of
 that physical quantity.

 Object must include a non-nil physical_quantity, i.e. a name or
 description of the physical quantity represented.