MRGT needs to process CTexts that specify synonyms

[RieksJ]

Issue essif-lab/framework#144 states that synonyms of terms need to be supported. The idea (that is currently being documented) specifies a different way in which an MRG is generated (that will be documented here, but the PR has currently not been created for that).

What will be new is that curated texts can specify a synonym of another term, for which the MRG entry must be generated in a way that differs from the current way. Basically, this will be the following steps:

• create a new MRG entry
• copy the front-matter of the term of which it is a synonym (or the MRG-entry in case that term comes from a different scope or version)
• copy the front matter of the curated text (that specifies the synonym) into the MRG-entry, overwriting possibly already existing fields
• discard the fields scopetag, locator etc (as specified in the table), and (re)create them as is also currently already done

This issue can be closed if the MRGT has such functionality