Port to ROS2 Galactic

README.md

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-# fuse
+# fuse - ROS2 port in progress
 
 The fuse stack provides a general architecture for performing sensor fusion live on a robot. Some possible applications
 include state estimation, localization, mapping, and calibration.

doc/Optimizers.md

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+# Optimizers
+The Fuse Optimizers are the top level executable structures\
+The Optimizer is launched by ROS, and instantiates additional ROS nodes to service the various sensor calls and other compute tasks
+
+A Fuse Optimizer chooses Constraints and Variables to insert or remove from the graph at run-time to regulate the amount of processing power and memory required to solve the graph.\
+During startup, an Optimizer reads configuration and loads sensor_models, motion_models, and publisher plugins, then waits for callbacks from each plugin.
+
+The plugins create Variables or Constraints, and submit Transactions to the optimizer in a callback.\
+Transactions are not carried by the ROS pub-sub model; the data-types of the Constraints and Variables are not well-defined as ROS messages, so they are carried by a callback wrapper where the callback can be processed by the ROS executors without serialising or transforming the data.
+
+The Optimizer decides when to perform an optimisation on the assembled graph, and submits the solved graph back to the plugins in another transaction.
+
+# Optimizer Base class
+This class provides plugin loader conventions, ROS parameter handling, and a few other helper functions that perform the basic administration of an optimizer.
+
+
+## Batch Optimizer
+The Batch Optimizer allows the graph to grow without limit, regularly solving the state.\
+This is suitable for high-latency or off-line systems that need to link measurements that are separated by a large amount of time, such as SLAM loop-closures
+
+
+## Fixed-Lag Optimizer
+
+The fixed lag optimizer removes graph entries that are older than some time horizon.\
+This is suitable for local navigation tasks where long histories have little effect on the confidence of a pose in local space.
+
+The special-case where only one measurement is preserved in the sliding window, is identical to an Extended Kalman filter.
+
+## Fixed-Size Optimizer
+(not yet available)
+
+This filter implements a sliding window similar to the Fixed-Lag optimizer, but aims to prune the graph to some size limit.
+
+