NGmenuEntry()
 Returns the Entry (adress) of a menu choice
------------------------------------------------------------------------------

 Function:      NGmenuEntry()

 Syntax:        NGmenuEntry(i,j) --> nEntry

 Description:   This function returns the adress associated with a menu
                Choice (or label). using the adress one can load a specific
                entry using the NGshort() or NGlong() functions.
                See the NGtype() and Overview entries for more information
                on Adresses.

 Returns:       a Number indicating the adress of this specific menu
                choice. i and j defined as in NGmenuLabel().