Wellsmanager: use new parser for WCONINJE and WCONPROD

In this PR the WellsManager class is updated to use the new Parser for (the content of) the WCONINJE and WCONPROD keywords. Observe that this PR requires a bleeding edge parser, i.e. https://github.com/OPM/opm-parser/pull/108 must hvae been applied before this will build.