Add currentControls() field to well state class.

This is done since the solver will need to be able to switch well controls
during Newton iterations. The current control specified in the Wells struct
will be used as default and initial value for currentControls().
Use currentControls() field of well state class.

Now the well state is consulted for the controls to use when assembling
the well control equations, instead of the (immutable) wells_ struct.

Also, added dummy implementation of updateWellControls().
Implemented updateWellControls().