Use well controls api

This PR will use the well_controls_get_xxx() and well_controls_set_xxx() functions in: https://github.com/OPM/opm-core/pull/454. 

Do make a careful check of the comments in 427f559 - I have tried to translate the code as verbatim as possible, and that gave:

    well_controls_iget_distr( wc , 0 )

but looking at the surrounding code I wondered if it should not be:

    well_controls_get_current_distr(wc)

instead?