Introduce explicit DummySolver for PETSc

PETSc seems pickier about matrices with nothing on the diagonal than the
other solvers. This `DummySolver` is an attempt to work around that,
although I'm not clear what the `DummySolver` is attempting to resolve in
the first place.

Addresses usnistgov#644

fipy/solvers/petsc/__init__.py

@@ -3,10 +3,11 @@
 from fipy.solvers.petsc.linearGMRESSolver import *
 from fipy.solvers.petsc.linearBicgSolver import *
 from fipy.solvers.petsc.linearCGSSolver import *
+from fipy.solvers.petsc.dummySolver import *
 
 DefaultSolver = LinearGMRESSolver
 DefaultAsymmetricSolver = LinearGMRESSolver
-DummySolver = DefaultSolver
+DummySolver = DummySolver
 GeneralSolver = DefaultSolver
 
 __all__ = ["DefaultSolver",

fipy/solvers/petsc/dummySolver.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python
+
+## -*-Pyth-*-
+ # ###################################################################
+ #  FiPy - Python-based finite volume PDE solver
+ # 
+ #  FILE: "dummySolver.py"
+ #
+ #  Author: Jonathan Guyer <guyer@nist.gov>
+ #  Author: Daniel Wheeler <daniel.wheeler@nist.gov>
+ #  Author: James Warren   <jwarren@nist.gov>
+ #    mail: NIST
+ #     www: http://www.ctcms.nist.gov/fipy/
+ #  
+ # ========================================================================
+ # This software was developed at the National Institute of Standards
+ # and Technology by employees of the Federal Government in the course
+ # of their official duties.  Pursuant to title 17 Section 105 of the
+ # United States Code this software is not subject to copyright
+ # protection and is in the public domain.  FiPy is an experimental
+ # system.  NIST assumes no responsibility whatsoever for its use by
+ # other parties, and makes no guarantees, expressed or implied, about
+ # its quality, reliability, or any other characteristic.  We would
+ # appreciate acknowledgement if the software is used.
+ # 
+ # This software can be redistributed and/or modified freely
+ # provided that any derivative works bear some notice that they are
+ # derived from it, and any modified versions bear some notice that
+ # they have been modified.
+ # ========================================================================
+ #  
+ # ###################################################################
+ ##
+
+__docformat__ = 'restructuredtext'
+
+from fipy.solvers.petsc.petscSolver import PETScSolver
+
+__all__ = ["DummySolver"]
+
+class DummySolver(PETScSolver):
+
+    """Solver that doesn't do anything.
+    
+    PETSc is intolerant of having zeros on the diagonal
+    """
+
+    def _solve_(self, L, x, b):
+        pass