pass a reference to the UnstructuredGrid to EclipseWriter instead of a shared_ptr

a shared_ptr might delete the grid object as soon as the shared_ptr
thinks that the object is unused. This happens as soon as there are no
other shared_ptr which were derived from the original one
anymore. Since this is a C object with plenty of raw pointers to it,
potentially deleting it is not a good idea...

(the current code works as well because the OPM 'share_obj' utility
function actually works around this by explicitly instructing
'shared_ptr' not to delete the object, but IMHO using this function
only makes it more complicated than neccessary.)

Note that this patch requires OPM/opm-core#560