Benchmark addon allows printing results as json file.

[martinfelis]

• Should enable tracking of performance regressions.
• Existing dynamics and contacts benchmarks supported (not yet: kinematics benchmarks)

[mkudruss]

Hi Martin,
thanks for the contributions. Json output will be very helpful in the future for performance regression analysis. Here are some general comments on your changes:

• CMakeLists.txt <- No comments.
• SampleData.h <- No problem with the additions but the file needs some reformatting to pass RBDL style guide. Spacing around ctor, dtor, etc. would make it better readable. Consider doing the changes for this file.
• benchmarks.cc <- Code is slightly refactored. The execution of benchmarks should be more generalized in the future for better maintainability. Same holds true for the manual composition of the json output.
• SimpleMath.h <- Nice that you added the eigen3 operators for simple math. Indeed helpful. Consider adding some "consts" when for read only variables.
• rbdl_version.cc <- No problem with the code but needs some reformatting to pass RBDL style guide. This is also true for your changes. Consider making the changes for the whole file.

[mkudruss]

@paulmanns will you perform the review or can we close the PR?

[paulmanns]

Approved.