Clean up Ordering + headers

[dellaert]

...and add test with disconnected graph.

[dellaert]

@akshay-krishnan the test I added passes. Can you share your failing test?

[akshay-krishnan]

This was my test, I did not EXPECT anything here, but was hoping to not crash when constructing ordering.

#ifdef GTSAM_SUPPORT_NESTED_DISSECTION
TEST(Ordering, metisBrokenGraph) {

  SymbolicFactorGraph symbolicGraph;

  symbolicGraph.push_factor(0);
  symbolicGraph.push_factor(0, 1);
  symbolicGraph.push_factor(1, 2);
  symbolicGraph.push_factor(3);

  Ordering metis = Ordering::Metis(symbolicGraph);
  const vector<int> xadjExpected{0, 1, 3, 4}, adjExpected{1, 0, 2, 1};
}
#endif
/* ************************************************************************* */

It did not fail, but printed the message here:

gtsam/gtsam/inference/Ordering.cpp

Line 242 in fae071d

std::cout << "METIS failed during Nested Dissection ordering!\n";

It is similar to the test you have added. Did you make any changes to the implementation? I dont see any.