Issue when computing pillar contacts

When using `pillars` to compute hc-contacts for fields with both gas and oil, only pillars that contains both phases are kept in the final result.

This comes from the default beaviour of pandas merge.
Setting `pd.merge(owc, goc, how="outer")` below will fix the issue.
https://github.com/equinor/res2df/blob/546eb94c42fd79d6c4940ffea48140dd95283f4d/res2df/pillars.py#L326

Open res2df/pillars.py around line 326 and inspect the merge of owc and goc. Verify the pillar-contact computation for fields containing both gas and oil, then confirm that the resulting data retains pillars present in either phase rather than only pillars present in both.

Written by the indexing model from the issue text.