Move individuals' stats attributes to new class IndivStats

src/libaevol/ae_individual.cpp

src/libaevol/ae_individual.h

src/libaevol/ae_individual_X11.cpp

src/libaevol/ae_individual_X11.h

src/libaevol/ae_replication_report.cpp

src/libaevol/ae_replication_report.h

src/libaevol/indiv_stats.h

src/libaevol/param_loader.cpp

src/libaevol/world.cpp