Add a Null Solver that has no action

src/PoissonSolvers/CMakeLists.txt

@@ -4,6 +4,7 @@ set (_SRCS
 set (_HDRS
     PoissonCG.h
     Poisson.h
+    NullSolver.h
 )
 
 if (ENABLE_FFT)

src/PoissonSolvers/NullSolver.h

@@ -0,0 +1,49 @@
+//
+// Class NullSolver
+//   Dummy solver which can be used when we require no action
+//   to be done on our LHS in the simulation while keeping the
+//   software design of the PIC framework in-tact.
+//
+
+#ifndef IPPL_NULL_SOLVER_H
+#define IPPL_NULL_SOLVER_H
+
+#include "Poisson.h"
+
+namespace ippl {
+
+    template <typename FieldLHS, typename FieldRHS>
+    class NullSolver : public Poisson<FieldLHS, FieldRHS> {
+    public:
+        using Base = Poisson<FieldLHS, FieldRHS>;
+        using typename Base::lhs_type, typename Base::rhs_type;
+
+        // constructors
+        NullSolver()
+            : Base() {}
+
+        NullSolver(rhs_type& rhs) {
+            using T = typename FieldLHS::value_type::value_type;
+            static_assert(std::is_floating_point<T>::value, "Not a floating point type");
+
+            Base::setRhs(rhs);
+            this->setDefaultParameters();
+        }
+
+        NullSolver(lhs_type& lhs, rhs_type& rhs)
+            : Base(lhs, rhs) {}
+
+        void solve() override {
+            // Overwrite the RHS (source rho) with the solution (potential phi), which is 0
+            *(this->rhs_mp) = 0.0;
+
+            // The gradient of the potential, which is the E-field, is also 0
+            if (this->lhs_mp != nullptr) {
+                *(this->lhs_mp) = 0.0;
+            }
+        }
+    };
+
+}  // namespace ippl
+
+#endif