Add a facility to easily access a local operator
Currently a LocalVariationalOperator is something really deep in the implementation of the library, and there is no easy way to access it (most objects required for its construction are themselves fairly internal ones).
The operators are thus tested at a global level, which is good enough for most of the cases.
However, for more subtle ones (such as the MITC4/9 policy introduced for non shell operators) it is really awkward not to be able to test them more directly, and the global operators can't yet be tested for some (MITC4 - see #1680 (closed)).
As I want to refactor MITC4 (in the review of #1664 (closed)), I need to be able to test I don't break anything - but the test involves a type of element not yet present. So I need a facility to provide access to the local operator without having to write too much code just to build it; the more straightforward is to provide a dedicated function in GlobalVariationalOperator.