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 think if (polymer.m_polymerConcentration > 0) is not a good criterion

Sure - that was just a suggestion. I guess the call could be unconditional?

>we should set a negative value or special value to identify that this injector is not a polymer injector.

No we should not - we should have something like `bool m_polymerInjectionActive`, special values like '-1' to signify logical behavior is a seed of misunderstandings.