Fix massflow/mesh can not use 'delete_atoms' with fix multisphere
It appears to be a huge inconvenience if 'fix massflow/mesh' cannot use 'delete_atoms' with 'fix multisphere', as the error shows below:
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ERROR: Fix massflow/mesh (id massflow): can not use 'delete_atoms' with fix multisphere/* (../fix_massflow_mesh.cpp:304)
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