Fix [EDUCATOR-338]: Errors: "'float' object is not callable"

Found out that the relevant Davidson courses use plenty
of max_attempts, but no max_scores. Used `grep`.

Resolves https://openedx.atlassian.net/browse/EDUCATOR-338 where
this XBlock was overriding the `max_score()` which edX grading
infrastructure expects as a method on the block.

We rename `max_attempts` to `maximum_attempts` to keep style consistent.

Initial implementation

It's not working for these fields, while it works for all other fields.  This
seems to be a bug in xblock-utils.

According to the docstring of the XBlock class, subclasses are not allowed to
override __init__().  (Seriously?)