Addition of VanHove in ddMd and modifications in ddMd/diagnostics

1. VanHove analyzer added in ddMd/analyzers
2. Potential energy in OutputEnergy now includes external energy if INTER_EXTERNAL is on
3. StructureFactor and StructureFactorGrid in ddMd/analyzers now output data at regular intervals through the simulation run
4. Addition of INTER_EXTERNAL methods- computeExternalEnergy() and externalEnergy() in ddMd/Simulation.cpp
5. loadParameters() and saveParameters() methods added to ddMd/potential/external/ExternalPotentialImpl.h
6. Similar changes in inter/extenal/PeriodicExternal.h