Implemented support for simple container-child record links using the hi...

...erarchy fields.

@demiankatz, would this be something that could be considered in the core VuFind?

A couple of remarks:

1.) SolrDefault gets the plugin manager since I feared instantiating Solr possibly unnecessarily would be too heavy.

2.) Linking to container uses hierarchy_parent_id regardless of the setting. I suppose that if you have both container_title and hierarchy_parent_id it would make sense to use the link automatically, but it could be made dependent on the setting if you think it's too risky.

3.) While we'd use this mostly for component parts, I tried to use generic names for settings etc. so this could be used for other parent-child relationships too.