EclipseWriter: work around a GCC 4.4 smart_ptr bug

maybe it is not a bug but a slightly 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...