hierarchical XML config files

[rheiland]

A reminder that we still have some hierarchical config files, rather than the "flattened" syntax that the Studio requires. At least these, it seems:

~/dev/PhysiCell_v1.12.0/sample_projects$ grep parent */config/*.xml
cancer_immune/config/PhysiCell_settings.xml:		<cell_definition name="immune cell" ID="1" parent_type="cancer cell">
celltypes3/config/PhysiCell_settings.xml:		<cell_definition name="A" ID="1" parent_type="default">
celltypes3/config/PhysiCell_settings.xml:		<cell_definition name="B" ID="2" parent_type="default">
celltypes3/config/PhysiCell_settings.xml:		<cell_definition name="C" ID="3" parent_type="default">
pred_prey_farmer/config/PhysiCell_settings.xml:		<cell_definition name="prey" ID="1" parent_type="farmer">
pred_prey_farmer/config/PhysiCell_settings.xml:		<cell_definition name="predator" ID="2" parent_type="farmer">
virus_macrophage/config/PhysiCell_settings.xml:		<cell_definition name="macrophage" parent_type="epithelial cell" ID="1">

The Studio's /config dir has flattened versions, but we should verify they reproduce the same results.