Fixes missing return value in non-void function of WellsManager.

gcc warned about the following

/home/mblatt/src/dune/opm/opm-core/opm/core/wells/WellsManager.cpp: In function ‘std::array<long unsigned int, 3ul> WellsManagerDetail::directionIndices(Opm::CompletionDirection::DirectionEnum)’:
/home/mblatt/src/dune/opm/opm-core/opm/core/wells/WellsManager.cpp:191: warning: control reaches end of non-void function

To calm it I introduced a throw clause after the switch statements. Thus adding a new
enum value will raise a warning on smart compilers, hopefully.
Merge pull request #684 from blattms/fixes-missing-return-value-in-wellsmanager

Fixes missing return value in non-void function of WellsManager.
[cleanup] Removes a superfluous if-check of the well state.

There were to identical if statements and the second one was followed
by an else branch. While in this case (if statement just throws) it is not
a bug, this commit cleans up one of the if statements.
Merge pull request #687 from blattms/remove-superlfluous-if-in-wellstate-check

[cleanup] Removes a superfluous if-check of the well state.
EclipseWriter: add possibility to write out the transmissibilities which are used by the simulator

the quantities are attributed to the positive faces of logically
Cartesian cells. I have not found a way to visualize the quanties in
the INIT file using ResInsight, so this is written once per timestep
for now...