add new polymerInflow constructor, get polymer injection rate from schedule section

With the new parser and timer, the old way to get polymer injection rate never work, and only pick up the value in the last keyword ``WPOLYMER``, this PR should solve the problem, it's related to PR https://github.com/OPM/opm-parser/pull/368 and issue https://github.com/OPM/opm-parser/issues/357. Please hold on after PR https://github.com/OPM/opm-parser/issues/357 is merged.
Well - the whole point about the `EclipseState` object is that it has internalized the *information* in the keywords, so it should not be necessary to access the raw keywords and records any longer. Just use the timeStep overload of `Schedule::getWells()` and you should be ready to roll:

```C++
std::vector<WellPtr> wells = schedule.getWells(wellNamesPattern);
  for (auto wellIter = wells.begin(); wellIter != wells.end(); ++wellIter) {
  WellPtr well = *wellIter;
  if (well->isInjector(timeStep)) { 
     WellPolymerProperties polymer = well->getPolymerProperties(currentStep);
     if (polymer.m_polymerConcentration > 0)
        wellPolymerRate_.insert(std::make_pair<std::string, double>(wellName, polymer.m_polymerConcentration));
  }
}
```