__str__ not always working

[Michael-E-Rose]

The str dunder for each class prints out some basic information. However, depending on the view, not all information is present, resulting in an ugly Exception.

For example print(str(AbstractRetrieval("2-s2.0-85166620539", view="META"))) doesn't work.

Some tasks:

• [x] Find out which class-view combinations do not work
• [x] For each, decide whether to provide fewer information in the __str__ or skipt it altogether (i.e., return None)
• [x] For each class where __str__ will be shut off for at least one view, update the documentation

[nils-herrmann]

There are 4 class-view combinations that trow an error:

nr	class	view	query	error_description
1	AbstractRetrieval	META	scopus_object = AbstractRetrieval('S092427162100068X', view="META")	'@id'
2	AbstractRetrieval	REF	scopus_object = AbstractRetrieval('S092427162100068X', view="REF")	'NoneType' object is not subscriptable
3	AuthorRetrieval	LIGHT	scopus_object = AuthorRetrieval('7004212771', view="LIGHT")	'NoneType' object is not subscriptable
4	AuthorRetrieval	METRICS	scopus_object = AuthorRetrieval('7004212771', view="METRICS")	'NoneType' object is not subscriptable