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.
Sorry for make too many comments not at the same time. I need to familiar with parser ...
I think ``if (polymer.m_polymerConcentration > 0)`` is not a good criterion, since sometimes we need to set polymer rate to 0 to let the simulator know polymer injection is stopped, this is what eclipse does, so in struct ``WellPolymerProperties``, we should set a negative value or special value to identify that this injector is not a polymer injector.