implement merge/update

ConstantLibrary should be allowed to merge/update itself with new values (for instance a custom set of mass values for Elements). This should update indicies and collections WITHOUT overwriting the index/collection objects... such that:

  SOME_INDEX = Element.library.collect :index {...}
  SOME_INDEX  # => the current :index
  
  Element.library.update ...

  SOME_INDEX  # => the updated :index