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.
PVT properties: allow them to be temperature dependent

Note that this patch does not introduce any real temperature
dependence but only changes the APIs for the viscosity and for the
density related methods. Note that I also don't like the fact that
this requires so many changes to so many files, but with the current
design of the property classes I cannot see a way to avoid this...