Add possibility to have multiple control variables

[luisaforozco]

In some cases, the control variables (system.ctrl_data) are not 1 dimensional.
They could treated in the same way as observations (system.obs_data) are i.e. list instead of single value.

Also in this same issue: Should we update the documentation to make it clear that for both system.ctrl_data and system.obs_data the last dimension is time?