Merge remote-tracking branch 'origin/ravi/charged_particles' into ravi/charged_particles

apps/showcases/ChargedParticles/ChargedParticles.cpp

apps/showcases/ChargedParticles/ErrorNorms.h

apps/showcases/ChargedParticles/PostProcessingUtilities.h

...articles/parameterfiles/ForceValidationSingleParticle.prm

...wcases/ChargedParticles/parameterfiles/singleParticle.prm

.../ChargedParticles/parameterfiles/testIterativeSolvers.prm

...ChargedParticles/poisson_solver/DirichletDomainBoundary.h

...articles/poisson_solver/ParallelCGFixedStencilIteration.h

...showcases/ChargedParticles/poisson_solver/PoissonSolver.h

...s/ChargedParticles/poisson_solver/PotentialValidation.cpp

.../ChargedParticles/poisson_solver/TestIterativeSolvers.cpp