Parse XML/MEI for work data sets

[dikastes]

Currently, only MerMEId previews are fetched as work datasets. Instead, MEI should be used as a base and data should be presented in a customised view.

[dikastes]

In the issue-2 branch, externally parsed JSON data (see Nathanael's converter) is commited to the searchengine.

The next step will be to include the conversion process to liszt_common and convert MEI to JSON in liszt_catalograisonne.