Refactoring for Python interface

- Moved TRowIterator and TRowIteratorWithRemove outside TTable
- Moved TPredicate enums outside TPredicate
- Created method to transform node property (THash) to TTable (Float properties only, to be continued)
- Order operation allows descending order
- Created getter for TTable Schema
Merge branch 'master' of https://github.com/snap-stanford/snapr