Install robots directory

[eborghi10]

This PR adds the robots directory which is missing is you try to install the package.

[codecov]

Codecov Report

Merging #96 (bd72dc2) into master (f1d4fda) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   69.65%   69.53%   -0.12%     
==========================================
  Files          32       32              
  Lines        1717     1717              
==========================================
- Hits         1196     1194       -2     
- Misses        521      523       +2     

Impacted Files	Coverage Δ
include/moveit_grasps/state_validity_callback.h	53.84% <0.00%> (-7.70%)	⬇️
src/two_finger_grasp_scorer.cpp	62.50% <0.00%> (-3.13%)	⬇️

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