Refactor/observation buffer

[kfabian]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses
Primary OS tested on	Ubuntu 22.04
Robotic platform tested on
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

• Changed the Observation class to store the value of the point cloud instead of a ptr to it, which simplified the class because there is no need for user implemented destructor and copy constructor anymore.
• Changed the ObservationBuffer to store a shared_ptrs of the Observations so that no deep copies of the point clouds are made when querying the buffer

Description of documentation updates required from your changes

Description of how this change was tested

• Existing tests for costmap
• Added basic unit test for ObservationBuffer

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Future work that may be required in bullet points

• The ObservationBuffer has lock and unlock member functions. In my opinion this error prone, the ObservationBuffer should either be thread safe on its own (by not exposing the mutex, but locking it in the implementation when needed) or it should not have the mutex member at all and the calling code would be responsible.

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[kfabian]

The previous implementation was assuming that the observation_list_ was ordered, but I think that is not guaranteed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
...costmap_2d/include/nav2_costmap_2d/observation.hpp	100.00% <100.00%> (+37.50%)	⬆️
..._2d/include/nav2_costmap_2d/observation_buffer.hpp	100.00% <ø> (ø)
...tmap_2d/include/nav2_costmap_2d/obstacle_layer.hpp	100.00% <ø> (ø)
...tmap_2d/plugins/costmap_filters/costmap_filter.cpp	89.33% <100.00%> (ø)
nav2_costmap_2d/plugins/obstacle_layer.cpp	82.05% <100.00%> (-0.12%)	⬇️
nav2_costmap_2d/plugins/voxel_layer.cpp	94.82% <100.00%> (ø)
nav2_costmap_2d/src/observation_buffer.cpp	98.59% <100.00%> (+8.05%)	⬆️

... and 2 files with indirect coverage changes