fix compiler warnings and rename CF to ConnectionTransmissibilityFactor

for the compiler warnings, see e.g.

https://opm-project.org/CDash/viewBuildError.php?type=1&buildid=27777

the CF to ConnectionTransmissibilityFactor rename makes things much easier to
understand for people who are not intimately familiar with the wells
code.
Merge pull request #300 from andlaus/fix_warnings

fix compiler warnings and rename CF to CompletionFactor