Perform system tests

Assuming heterogeneity has been compiled, run it using a config file using a single thread:

~/git/PhysiCell/tests/system$ ../../heterogeneity config_hetero_1core.xml

then verify the final.svg file is what we expect:

~/git/PhysiCell/tests/system/output_heterogenity$ diff final.svg final_1core_240min.svg
...

The only difference should be the amount of time used in the computation.