Fixes & improvements for deleting markers

[wyattrees]

• Adds ability to delete all markers in a given namespace
• deleting all markers was previously broken, as it did not have an empty namespace, which is expected by rviz
• added getters for marker ID's so deleteMarker can be called with an ID

[codecov[bot]]

Codecov Report

Merging #188 (76a30b8) into foxy-devel (dcc457a) will decrease coverage by 0.29%. The diff coverage is 3.57%.

@@              Coverage Diff               @@
##           foxy-devel     #188      +/-   ##
==============================================
- Coverage       19.49%   19.20%   -0.30%     
==============================================
  Files               5        5              
  Lines            1749     1776      +27     
==============================================
  Hits              341      341              
- Misses           1408     1435      +27     

Impacted Files	Coverage Δ
include/rviz_visual_tools/rviz_visual_tools.hpp	16.00% <ø> (ø)
src/rviz_visual_tools.cpp	16.00% <3.57%> (-0.29%)	:arrow_down:

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