Circles end.xml exception

[ptheywood]

Running the circles_spatial3D example twice with visualisation enabled results in terminataion due to the end.xml already exsits error.

This is not ideal for a visualisation example.

./bin/Release/circles_spatial3D 
100.00% Drift correct
terminate called after throwing an instance of 'flamegpu::exception::FileAlreadyExists'
  what():  /home/ptheywood/code/flamegpu/FLAMEGPU2/src/flamegpu/simulation/Simulation.cu(131): File 'end.xml' already exists, in Simulation::exportData()

We should probably gracefully handle the end.xml already existing in our examples. Probably applies to both circles examples?