Fix variable names in Loader.assignIndices

This function can be applied to any type of class "Element" but its
internal variables used to refer to a list of tuples containing these
elements and their names as "nodes". Moreover it built a list of name to
indices tuples referring to it as "na" (which is not very clear).

Thus nodes becomes name_element, and na name_idx.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>