It seems to me that if I had an EquationSystems object with 2 systems,
System 0 and System 1, then I deleted System 1, then I added a new
system, that new system would also get numbered System 1...
I ask this question not because I need delete_system() to work
correctly, but because I'm planning to CVS commit a change to
EquationSystems::solve() that would break delete_system() even
further... but before I do so, I'd like some reassurance that it's a
pointless function that nobody uses.