Diagnostics interface and projected energy density

I added an abstract base class Diagnostics. Any diagnostics inherits from it and has to implement the execute() method, see for example ProjectedEnergyDensity. After setting up the simulation, one has to call Simulation.executeDiagnostics() to perform all diagnostics like e.g. calculate the projected energy density.

The main programm evolves a Gaussian pulse in time and writes the projected energy density to a file after every time step to analyse the evolution.

I also implemented a test to check if the total energy is preserved over time.