adding gradient layer costmap2d plugin tutorial

[AlexeyMerzlyakov]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#1533
Primary OS tested on	Ubuntu
Robotic platform tested on	Gazebo simulation of TB3 waffle

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Description of contribution in a few bullet points

• Adding tutorial: Writing a new Costmap2D plugin: gradient layer

[codecov]

Codecov Report

Merging #1541 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1541      +/-   ##
==========================================
+ Coverage    37.2%   37.24%   +0.03%     
==========================================
  Files         226      226              
  Lines       11807    11807              
  Branches     5135     5135              
==========================================
+ Hits         4393     4397       +4     
+ Misses       4014     4013       -1     
+ Partials     3400     3397       -3

Flag	Coverage Δ
#project	37.24% <ø> (+0.03%)	⬆️

Impacted Files	Coverage Δ
nav2_navfn_planner/src/navfn.cpp	46.33% <0%> (ø)	⬆️
nav2_map_server/src/map_server.cpp	42.85% <0%> (ø)	⬆️
nav2_util/test/test_actions.cpp	42.66% <0%> (+0.45%)	⬆️
nav2_voxel_grid/src/voxel_grid.cpp	67.28% <0%> (+2.8%)	⬆️

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[SteveMacenski]

Small updates, this looks really good. My requests are just grammar and formatting. For anything I could just give a suggested edit, you can merge directly in browser. I couldn't do that for the tables though.

Looks great. After those updates we can merge this in. Thanks for making a very thorough tutorial, this will do alot of good. You explained things very well.

[SteveMacenski]

You've inspired me to get to writing a controller and planner plugin tutorial as well. I'll play around with that this weekend

[AlexeyMerzlyakov]

Thank you for very positive feedback! Everything is seems to be done.