C API -- Variable lists

• Obtain variable list handle:
  FCM_VARLIST vl_handle = fcm_lookup_varlist(fcm, "@name")
• Read model data:
  fcm_fetch(model_handle, vl_handle, buf, len)
• Write model data:
  fcm_store(model_handle, vl_handle, buf, len)
• buf: pointer to array of len doubles
• model_handle: model instance handle obtained from fcm_create()