AutoFilter Improvements

[oleibman]

Fix #2378. The following changes are made:

• NotEqual tests must be part of a custom filter. Documentation has been changed to indicate that.
• Method setAndOr was replaced by setJoin some time ago. Documentation now reflects that change.
• Documentation to indicate that string filters are not case-sensitive, same as in Excel.
• Filters testing against numeric value now include a numeric test (not numeric for not equal, numeric for all others).
• String filter had previously treated everything as a test for "equal". It now handles "not equal" and the variants of "greater/less" with or without "equal".
• Documentation correctly stated that no more than 2 rules are allowed in a custom filter. Code did not enforce this restriction. It now does, throwing an exception if an attempt is made to add a third rule.
• Deleted a lot of comments in Rule.php to make it easier to see what is not yet implemented (between, begins with, etc.). I may take these on in future.
• Added a number of tests for the new functionality.

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?