Set self href on items and children when adding

[matthewhanson]

Related Issue(s): #
#284

Description:
When an existing catalog and an item or child is added to it, the self_href was not set which meant that they had to either be set manually, or a call to normalize_hrefs would be needed. This would also happen if generate_subcatalogs was used....all the subcatalogs and items added will need to be normalized.

However, since the catalog already exists, this should be unnecessary. Given a layout strategy, a self href can be determined and set, if the parent has a self href.

Note the only case where a parent does not have a self href is when the catalog is first created in memory and has not yet been saved. In that case a call to normalize is needed

PR Checklist:

• Code is formatted (run scripts/format)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.