makd PolymerProperties handle PLYSHEAR keyword.
make the member functions return a const reference.
return the value by linear interpolation.
Removed Wellsmanager constructor which used old EclipseGridParser instance.
Merge pull request #55 from joakim-hove/remove-old-WellsManager

Removed deprecated Wellsmanager constructor
let shear vef return derivative.
Implement low-level init() routines

These are needed by the initialisation routines of the refactored
opm-core (PR OPM/opm-core#518) that introduces support for parallel
grids.
Merge pull request #56 from bska/fallout-from-refactoring

Implement low-level init() routines
Merge pull request #52 from qilicun/read_plyshear

make PolymerProperties handle PLYSHEAR keyword.