Clean up the SimpleSOL solver and structure it more like H3LAPD (#227)

* Add missing header guard.

* Rejig SimpleSOL such that the v_DoSolve override can be removed.

* Remove redundant DoDiffusion().

* Remove redundant SimpleSOL test.

* Inherit from UnsteadySystem rather than AdvectionSystem and remove some redundant functions.

* Remove virtual inheritance for SOLSystem, SolWithParticlesSystem.

* Remove commented code.

* Remove FluidInterface inheritance.

* Rejig SimpleSOL namespaces.

* Remove some unused functions.

* Remove some nektar-specific licence headers.

* Consistent header guards.

* Remove some unused functions.

* Consistent variable, function names, add some docstrings.

* Rejig get_flux_vector() - might address intermittent segfault?

* Tidy up SourceTerms class.

* Remove some 'using namespace...' directives.

* Add missing std:: qualifiers.

solvers/SimpleSOL/Diagnostics/mass_conservation.hpp

@@ -1,23 +1,21 @@
 #ifndef __SIMPLESOL_MASS_CONSERVATION_H_
 #define __SIMPLESOL_MASS_CONSERVATION_H_
 
+#include <LibUtilities/BasicUtils/ErrorUtil.hpp>
 #include <memory>
 #include <mpi.h>
 #include <neso_particles.hpp>
-using namespace NESO;
-using namespace NESO::Particles;
-
-#include <LibUtilities/BasicUtils/ErrorUtil.hpp>
-using namespace Nektar;
 
 #include "../ParticleSystems/neutral_particles.hpp"
 
 #include <fstream>
 #include <iostream>
 
+namespace LU = Nektar::LibUtilities;
+
 template <typename T> class MassRecording {
 protected:
-  const LibUtilities::SessionReaderSharedPtr session;
+  const LU::SessionReaderSharedPtr session;
   std::shared_ptr<NeutralParticleSystem> particle_sys;
   std::shared_ptr<T> rho;
 
@@ -27,10 +25,10 @@ template <typename T> class MassRecording {
   double initial_mass_fluid;
   int mass_recording_step;
   int rank;
-  ofstream fh;
+  std::ofstream fh;
 
 public:
-  MassRecording(const LibUtilities::SessionReaderSharedPtr session,
+  MassRecording(const LU::SessionReaderSharedPtr session,
                 std::shared_ptr<NeutralParticleSystem> particle_sys,
                 std::shared_ptr<T> rho)
       : session(session), particle_sys(particle_sys), rho(rho),