Removed cpos = ~cpos constructions for closing a well

This is a fallout from https://github.com/OPM/opm-core/pull/507

Closing wells was performed by:
```C++
cpos = ~cpos;
set_current_control( );
```

This fails hard when a well has been shut and has no valid controls. With this PR the `well_controls_shut_well()` is used. As an extension of the same theme the second commit introduces a change in the WellsManager: it will check that a well is open before throwing on an invalid control.

With these two commits the opm-core/WellsManager can load the "complete" NORNE Schedule file - with the following caveats:

  1. Many wells have control mode RESV in the WCONHIST keyword; as discussed previously we really do not know how to handle that in a sane manner. I have manually applied s/RESV/ORAT/g throughout the NORNE SCHEDULE file.

  2. We do not currently handle the 'STOP' well status, there was only one active 'STOP' status in the whole file, I changed that to 'SHUT'.

A small summary application is coming in a separate PR when this is merged.