EclipseWriter: work around a GCC 4.4 smart_ptr bug

maybe it is not a bug but the fact that GCC 4.4 uses a non-final c++-2011 spec. The problem is that GCC 4.4 does not implicitly convert `std::shared_ptr<$FOO>` to `std::shared_ptr<const $FOO>` which caused the recent Jenkins build errors at Statoil. Note that this problem only occurs with the output writer in conjunction with the old Eclipse parser, so OPM/opm-autodiff#105 also makes the problem disappear. The present patch addresses the root cause, though...