Overwrite __repr__ methods for MardiEntities

[eloiferrer]

New feature description in words:
I find the current implementation of repr in BaseEntity.py confusing to get a quick overview of an entity.
repr should return:

• English/German label
• English/German description
• Key-value pairs for statements and qualifiers if present.

TODOS:

• Rewrite repr in MardiEntities.py to implement the previous schema.

Checklist for this issue:
(Some checks for making sure this feature request is completely formulated)

• Participants in discussion have been invited as assignees
• All fields of the issue have been filled
• Example fields have been removed
• The main MaRDI project has been assigned to this issue