Rework of Input objects: InputVoltage/InputCurrent/InputFlux with operating point object

[EmileDvs]

The implementation of EEC circuits requires to setup voltage instead of current at the beginning of the simulation.

In PR #464, the new object for this purpose in InputVoltage, that replaces the previous InputElec. Input is the abstract class which is never used for simulation since there is no control variables in it, such as current or voltage. Then InputVoltage inherits from Input, InputCurrent inherits from InputVoltage and InputFlux inherits from InputCurrent, so that for example if flux is enforced as an input, the reference current/voltage/slip can still be given as metadata.

Besides, since PMSM and SCIM are driven with different parameters (Ud/Uq/Id/Iq for the first one, U0/I0/phi0/slip for the second one), a new OP object (Operating Point) is implemented to handle the complexity of both controls with two children classes OPdq and OPslip.

[EmileDvs]

Issue solved with #464