Create Class: Remove context settings in favour of schema-only non-context

[janezd]

Issue

Fixes #5499, assuming that weird behaviour is caused by context settings. (Which it almost certainly is.)

Description of changes

As per title, I replaced context settings with schema-only (non-context) settings. The only remaining context setting is the variable.

It gets even better. Rules were already an "internal context setting" with the variable as a context. That is, if the user sets the rules for one variable, switches to another, sets some rules, and switches back (s)he will get the rules for the former variable.

After removing context settings, this works as hinting. Load zoo, set some rules for type. Load Iris (rules disappear), set different rules. Go back to zoo and you get the rules back. This hinting is based on variable name.

If this is merged and documentation is updated, close #6961, too.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.03%. Comparing base (246474b) to head (feac944).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7231   +/-   ##
=======================================
  Coverage   89.03%   89.03%           
=======================================
  Files         335      335           
  Lines       74243    74258   +15     
=======================================
+ Hits        66099    66117   +18     
+ Misses       8144     8141    -3     

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