Add ==, !=, >, <, and string equality computed columns

[sc1f]

This PR adds and tests several new computed column operators:

• ==, !=, >, and < for comparing numeric columns
• is for comparing string columns

For numeric operations, when either value is null the comparator will return null. However, for strings, if either value is null the comparator returns false. This is due to an issue with null handling inside computed columns in general, which has been fixed in the new computed column API.

[texodus]

Thanks for the PR! Great test coverage!

Can you please take a look at the string comparison note when you get a chance?

[texodus]

Looks great! Thanks for the PR!