Check and fix all docstrings

[einbandi]

Things to consider:

• Add :class:, :meth:, etc. specifiers.
• Check indentation, especially in argument lists.
• Check again what the best practices are for type information.
• Check if methods use third person.

[einbandi]

Class versions of relation conversion methods hsould have same docstrings as functional ones.

[einbandi]

• exceptions.py
• loss.py
• models.py
• paradime.py
• relationdata.py
• relations.py
• transforms.py
• utils.py