Add representation logic to `VotingEnsemble`

[RaulFD-creator]

Is your feature request related to a problem? Please describe.
The VotingEnsemble cannot work isolated because it requires additional logic to compute the representations.

Describe the solution you'd like
Add a private method to compute the representations and perform any necessary pre-processing.

[RaulFD-creator]

Workaround for now (v.2.0.3) by keeping track in APML class of the RepEngines used, so that they can be more flexibly used to generate the representations to a VotingEnsemble