ENH: create sensible route for updating an existing catalog to a new release

[jsheunis]

The goal would be to have a command or commands that allow:

• updating any web assets to the latest release (html, js, css, etc)
• transforming metadata to a new structure if required
• adding updated schema docs if required
• transforming config files to a new structure if required
• what else?

[jsheunis]

If the latest version is installed, running datalad catalog-create -c <existing-catalog-location> --force will overwrite the following (wrt catalog root):

• assets
• artwork
• index.html
• README.md
• schema
• templates
• config.json

i.e. everything except for the metadata.

If the catalog is a git repo, a short term workaround to update only html+JS+CSS+other assets would be to run the command above and then only commit the changes that one wants to keep (ping @mslw).

However a more sensible option for future would be to have a datalad catalog-update command that implements parameterized update pipelines (e.g. all, assets-only, schema-only, etc)