Add NOT conditional operator 

I see not way to introduce `NOT` operator (not to be confused with `NOT IN`) in the `WHERE` part, otherwise than rewriting the query.
Example:
```sql
WHERE NOT (Industry = 'Agriculture' OR Industry = 'Biotechnology')
```
cannot be built in this framework; we need to rewrite it as:
```sql
WHERE Industry != 'Agriculture' AND Industry != 'Biotechnology'
```