Add metadata about implementations to entries

[Zahnentferner]

On this commit, I created a new metadata command for mentioning implementations of a proof system. The implementations are then listed in an index in the end of the Encyclopaedia.

We need to go through the existing entries and add provers that we already know.

We could also contact the authors, and ask them to tell us about their implementations or add them themselves.

[gisellemnr]

Here's a good list of implementation of modal systems: www.cs.man.ac.uk/~schmidt/tools/

[olaure01]

Corrected link:

Here's a good list of implementation of modal systems: www.cs.man.ac.uk/~schmidt/tools/