add a DEFAULT control mode for wells

the Norne dataset sets a control mode to 'DEFAULT'. At least for the
WCONPROD keyword, the eclipse manual does not allow this, but it says
that the repective field may be defaulted to '' if the well is shut
which will result in an undefined control mode. Let's thus just take
a random one of the implemented modes...

Note, that a different mode may be chosen to replicate eclipse
behavior or for better computational performance. Suggestions are
welcome...