Add the parameter `OptimizationProblemOptions.use_online_imu_extrinsincs_in_3d`. Set it to true by default to not change existing behavior.
Using online IMU extrinsics is not always desirable, and particularly during localization, we have determined that it is preferable to turn this feature off.
"scan" is better named "node" if it refers to a node in global SLAM.
"penalty factor" is renamed "weight" for consistency with other similar options.
"scans_per_accumulation" is renamed "num_accumulated_range_data" to match the code and since the accumulated data is not called "scan".