Add option to skip python bindings

[mjcarroll]

🦟 Bug fix

Addresses gazebosim/gz-cmake#300

Summary

Adds SKIP_PYBIND11 to allow for Python binding generation to be skipped.

Checklist

• Signed all commits for DCO
• Added tests
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #1106 (09eb94d) into sdf13 (78adf4c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            sdf13    #1106   +/-   ##
=======================================
  Coverage   86.70%   86.70%           
=======================================
  Files         125      125           
  Lines       15890    15890           
=======================================
  Hits        13778    13778           
  Misses       2112     2112           

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[scpeters]

@osrf-jenkins run tests please