fix compiler warnings and rename CF to CompletionFactor

for the compiler warnings, see e.g.

https://opm-project.org/CDash/viewBuildError.php?type=1&buildid=27777

the CF to CompletionFactor rename makes things much easier to
understand for people who are not intimately familiar with the wells
code. (if "CF" does not stand for "completion factor", this only
underscores the point. In this case, please add a comment what it
means then...)

Note: this PR must be merged sychronously with OPM/opm-core#650