Refactor processing of inequalities with oo [1/9]

fixes #7775
Refactor processing of inequalities with oo [2/9]

If we don't know the boundedness: _eval_is_positive/negative should
return None.

fixes #7798
fixes #7841
Refactor processing of inequalities with oo [3/9]

fix tests in test_sets.py
Refactor processing of inequalities with oo [4/9]

fix wester's tests
Refactor processing of inequalities with oo [5/9]

fix tests for inequalities.py
Refactor processing of inequalities with oo [6/9]

More tests fixed
Refactor processing of inequalities with oo [7/9]

Add import for oo
Refactor processing of inequalities with oo [8/9]

Fix physics/quantum tests due to simple bug
in Expr._eval_interval (check for NegativeInfinity singleton)

(Thanks to Aaron Meurer for idea)
Refactor processing of inequalities with oo [9/9]

Fix doctests in delta.py
Merge pull request #7861 from skirpichev/oo-ineq

Refactor processing of inequalities with oo