Easy-to-fix pylint issues in probnum.diffeq

[pnkraemer]

Context:
There is ongoing work on refactoring probnum to comply with the desired pylint configuration. See #523 for context.
Many of those refactorings should be very simple to resolve.

Details:
For the probnum.diffeq subpackage, these are

• line-too-long (some docstrings have too long lines)
• no-else-return
• no-else-raise

Once a message has been resolved, please remove the explicit pylint-ignore on the bottom of the file.

Some notes, if you would like to take this on:

• Have a look at the before opening a pull request.
• Please strongly consider resolving each message in a separate pull request. This makes the reviews go more smoothly.

[akapoor13]

Working on this today.