Is your feature request related to a problem? Please describe.
The documentation of Hazard and Impact states that the event_name attribute is supposed to be a list of strings. However, this is never enforced within the code base. In particular, some unit tests actually check if "mixed-type" lists work. In #894, we decided that

• a write method should not proceed if event_name contains other values than strings
• a read method should warn if event_name is not stored as strings and try to convert the values to strings.

Describe the solution you'd like
Apply the above decisions to all readers and writers of Hazard and Impact.

Describe alternatives you've considered

• Converting any event_name values to strings before writing might lead to nonsense, see Explicitly convert event_name to strings in Impact.from_hdf5 #894 (comment). Therefore, writing non-string event_name values should fail.

Additional context
None