this prevents GCC-7's address sanitizer from complaining about a
delete for a new of a different type. (i.e., the new is for the
derived class, but if the destructor is non-virtual only the dtor of
the base class is called if a pointer to the base class is deleted.)
finding this was quite some fun, mainly because it took a while what
the issue was.
We need it serveral places and all of them seem to have access to
NewtonIterationBlackoilInterface. This makes it natural to give access
to it and prevent users from having to forward it manually at several
places in the simulator driver.
