Apply clang format fixes to model.cpp.
Apply clang format fixes to model.cpp.

cellml/libcellml,MichaelClerx/libcellml,nickerso/libcellml,nickerso/libcellml,hsorby/libcellml,MichaelClerx/libcellml,cellml/libcellml,cellml/libcellml,cellml/libcellml,MichaelClerx/libcellml,nickerso/libcellml,hsorby/libcellml,hsorby/libcellml,hsorby/libcellml,nickerso/libcellml