Add space of closed curves starting at origin

[luisfpereira]

In #1904 we've deeply refactored discrete_curves.py. During this process, we've decided to remove the space of closed curves (ClosedDiscreteCurves), as it was not well defined. It is now time to bring it back.

How? Spatial velocities are computed using finite differences. It should suffice to make this computation "circular", i.e. add origin back to the array at the end if curve is closed.

Should I add a new space? A flag in DiscreteCurvesStartingAtOrigin (e.g. closed) should do the job, but if it becomes tricky to handle, we can add ClosedDiscreteCurvesStartingAtOrigin.

All the metrics defined for DiscreteCurvesStartingAtOrigin should work in the closed case.

Check if #1511 can be closed.