Introduce <disable_default_collisions> and <enable_collisions> tags

[Abishalini]

Ported from noetic-devel branch since this is required to port this PR to MoveIt2

[codecov]

Codecov Report

Merging #101 (ee1ee7f) into ros2 (6ebc388) will decrease coverage by 0.39%.
The diff coverage is 71.43%.

@@            Coverage Diff             @@
##             ros2     #101      +/-   ##
==========================================
- Coverage   68.21%   67.83%   -0.38%     
==========================================
  Files           3        3              
  Lines         607      634      +27     
==========================================
+ Hits          414      430      +16     
- Misses        193      204      +11     

Impacted Files	Coverage Δ
src/model.cpp	59.80% <55.18%> (-0.55%)	⬇️
src/srdf_writer.cpp	78.95% <82.15%> (-1.36%)	⬇️
include/srdfdom/model.h	100.00% <100.00%> (ø)

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