Possibility to initialize ecl_smspec_node directly

afaik not possible to read `SMSPEC` and get metadata without also reading `UNSMRY` (by using `EclSum`).
See e.g: [ecl_smspec_node init function](https://github.com/equinor/ecl/blob/1b671813f27bd73b2ecefb9152106cc8aa38ab1f/python/ecl/summary/ecl_smspec_node.py#L47-L49)

If you want to extract the union of metadata from many simulations, having to copy and process `UNSMRY` adds a lot of unnecessary overhead.

Would be nice to be able to make an object that only contains what is covered by ecl_smspec_node by reading a `SMSPEC` file directly.