Major changes:

- New square-root implementation for optimization and
  marginalization, giving faster optimization and numerically
  more stable marginalization. The square root solver is the new
  default, but the Schur complement based implementation is still
  available. (Implements the ICCV'21 paper.)

- The odometry estimator is now fully templetized and you can run
  in float or double. Default is float, which works well with the
  new square-root implementation and gives best runtimes.

- Batch evaluation scripts and documentation to reproduce the
  ICCV'21 experiments.

Additional changes:

- New options in VIO to marginalize lost landmark right away and
  not only when the frame is marginalized (enabled by default).

- small bugfix for keypoint patch extraction bounds

- basalt_vio: more logging for batch evaluation

- basalt_vio: better handling of closing the GUI while estimator is still running

- basalt_vio: new command line argument to ...