Introduce Support for Horizontal Completions and Net-To-Gross Ratios

This change-set generalises the calculation of the Peaceman index of a completion to account for horizontal completions and effects of net-to-gross ("NTG") ratios.  The change is contained in the implementation of the `WellsManager`, so all clients get these features without code change.

At this time the NTG factor is incorporated unconditionally if present in the input represented by `WellsManager`'s `eclipseState` constructor parameter.  We may want to make that policy configurable for clients.  The change-set uses features of OPM/opm-parser#295 and cannot be merged until that is available in opm-parser's master branch lest the build break.

This change-set supersedes earlier efforts and therefore closes #622, closes OPM/opm-autodiff#174, and closes OPM/opm-polymer#66.
There is no standard definition for pi (or tau, for that matter).