Terminate sweeps if they go too long

PETSc's LinearLUSolver presently doesn't converge, causing this example to
run forever.

Addresses usnistgov#644

examples/phase/binaryCoupled.py

@@ -514,12 +514,14 @@ def deltaChemPot(phase, C, T):
 >>> C.updateOld()
 >>> res = 1.
 >>> initialRes = None
+>>> sweep = 0
 
->>> while res > 1e-4:
+>>> while res > 1e-4 and sweep < 20:
 ...     res = eq.sweep(dt=dt, solver=solver)
 ...     if initialRes is None:
 ...         initialRes = res
 ...     res = res / initialRes
+...     sweep += 1
 
 >>> from fipy import input
 >>> if __name__ == '__main__':
@@ -604,8 +606,10 @@ def deltaChemPot(phase, C, T):
 ...     phase.updateOld()
 ...     C.updateOld()
 ...     res = 1e+10
-...     while res > 1e-3:
+...     sweep = 0
+...     while res > 1e-3 and sweep < 20:
 ...         res = eq.sweep(dt=dt, solver=solver)
+...         sweep += 1
 ...     if __name__ == '__main__':
 ...         viewer.plot()