Add a function to find a node from the chain by link name

[ssr-yuki]

I am not sure if the function name find_link is appropriate... Any comments are welcome.

Another candidate: find_by_link_name

[codecov[bot]]

Codecov Report

Merging #92 (d1b0ec3) into main (98c09f2) will decrease coverage by 0.36%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
- Coverage   68.79%   68.43%   -0.37%     
==========================================
  Files          14       14              
  Lines        1564     1565       +1     
==========================================
- Hits         1076     1071       -5     
- Misses        488      494       +6     

Impacted Files	Coverage Δ
src/chain.rs	59.70% <0.00%> (-1.48%)	:arrow_down:
src/funcs.rs	87.50% <0.00%> (-0.50%)	:arrow_down:
src/ik.rs	80.43% <0.00%> (ø)
src/urdf.rs	77.73% <0.00%> (+0.44%)	:arrow_up:

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