Cleaned the heterogeneity MPI example files. While compiling it gave an error due to custom.h file included in PhysiCell_cell.cpp (don't know why it was there) but it was written that it should be deleted later, so have deleted, now compiles successfully and runs.
Changed the name of script_hetero.sh to script_hetero_mpi.sh and re-wrote and organized the source code within it.