Move state-related code to a subdir

src/Makefile.am

src/one-body-sample.org

src/state/compute_state_dieterich_common.cc

src/state/compute_state_dieterich_common.hh

src/state/compute_state_dieterich_euler.cc

src/state/compute_state_dieterich_euler.hh

src/state/compute_state_ruina.cc

src/state/compute_state_ruina.hh

src/state/dieterichstateupdater.hh

src/state/ruinastateupdater.hh

src/state/stateupdater.hh