WellsManager: Support NTG and horizontal completions

This commit extends the feature set of the WellsManager to support
horizontal ("X" and "Y") completions and include the net-to-gross
ratio in the Peaceman index ("Completion Transmissibility Factor,
CTF") of a well completion.  The NTG factor is included if present
in the input deck represented by the "eclipseState".

There are two separate, though related, parts to this commit.  The
first part splits the calculation of Peaceman's "effective radius"
out to a separate utility function, effectiveRadius(), and
generalises WellsManagerDetail::computeWellIndex() to account for
arbitrary directions and NTG factors.  The second part uses
GridPropertyAccess::Compressed<> to extract the NTG vector from the
input if present while providing a fall-back value of 1.0 if no such
vector is available.

Note: We may wish to make the extraction policy configurable at some
point in the future.