dev: code improvements for DBLP search source

[geritwagner]

Feedback (from #308 )

• [ ] Documentation needs to be updated and enhanced -> more specifics, e.g. user inputs for search
• [ ] Update and enhance user interface (add advanced search parameters, e.g. yearspan, venue, etc.)
• [x] Extract code related to json to dict conversion to other classes for better clarity and readability: shorten dblp.py
• [x] Implement missing searchtypes (TOC & MD)