Fix SHUT wells
Shut wells are not added to the well list and thus not considered in the
simulator.

The shut well test in test_wellsmanager is modified to assert this
behaviour.

BUG: This change provokes an assert in the EclipeWriter as number of
wells in wellstate is different from number of wells in the schedule.
SHUT wells returns 0
Shut wells returns 0 for bhp and rates in the EclipseWriter
Make the comment in test_wellsmanager more precise
The comment now precisly says that Shut wells are not added instead to
the well list instead of removed.