DOC consistent objective description

There is several issue with the description of the objective in each benchmark:

- The latex equations are not rendered in the `README.rst`
- We need to maintain them at several places (benchmark README, list of objective in the doc, it would be nice to include in benchmark result page, ...)

A potential solution would be to have a small file `objective.rst` in the benchmark folder that describe the objective. We could include it when generating the doc, and use sphinx to convert it to HTML for the results and the README.

This requires significant developments effort so it is clearly not a priority.