Stable release before massive changes

This release includes new experiment features such as:

• constraint management
• multiobjective functions
• new optimizers
  This is a stable release before a transition phase in which we will refactor the instrumentation part of the package, allowing a lot more flexibility. These changes will unfortunately probably break some use code, and we expect a few bugs during the transition period.

See CHANGELOG for more about this release, and checkout Nevergrad users Facebook group for more information about upcoming changes.