Add option for parsing root joint type without world link

[jslee02]

The root joint type cannot be specified when the URDF doesn't contain "world" link. In that case, the URDF parser in DART assumes the root joint type as "free" joint, but some user could expect it to be "fixed" joint.

This PR makes that behavior configurable using flags.

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[codecov]

Codecov Report

Merging #1399 into master will increase coverage by 0.03%.
The diff coverage is 83.33%.

@@            Coverage Diff            @@
##           master   #1399      +/-   ##
=========================================
+ Coverage   57.27%   57.3%   +0.03%     
=========================================
  Files         366     366              
  Lines       27432   27442      +10     
=========================================
+ Hits        15711   15726      +15     
+ Misses      11721   11716       -5

Impacted Files	Coverage Δ
dart/utils/urdf/DartLoader.hpp	100% <ø> (ø)	⬆️
dart/dynamics/SharedLibraryIkFast.cpp	49.41% <0%> (ø)	⬆️
dart/utils/urdf/DartLoader.cpp	78.83% <89.28%> (+0.74%)	⬆️
dart/dynamics/WeldJoint.cpp	86.66% <0%> (+16.66%)	⬆️