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.
> I guess the call could be unconditional?

Yes, still need to identify polymer injection well, but anyway, that is a another way to get the target, maybe the refactor will be done in the near future. But this patch itself works fine.