Initialize well rates better.

Instead of making well rates zero for wells that are not controlled by
surface volume, we initialize them to a small value with the correct
sign (positive for injectors, negative for producers).
Commented choices in initialisation of well rates.
Assert to avoid future surprises.

Well types must be either INJECTOR or PRODUCER for now, if
we change this in the future, we should check this part of
the code as well.
Bugfix: || -> &&
Fix logic related to open/shut wells initialisation.
Merge pull request #508 from atgeirr/adjust-initial-wellrates

Adjustment to well rate initialisation.