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.
Maybe here we should use ``std::vector<WellPtr> wells = schedule.getWells()`` but it's better to know well name, since we use ``std::unordered_map`` to store polymer concentration, if not we should find another way to store these values.