Datasiphon

- added support for advanced ordering null values in `SQL` backend using nulls last and nulls first methods
- adjusted operators `eq` and `ne` - correct handling of bool type columns and `None` values

- adjusted dumping expression to avoid merging junctions which would result in incorrect query - now dumped into array like dict

- Refactored project - core structure moved to `core` module, sql backend moved to is now in `sql_filter` module
- Rewritten tests
- Modified structures and objects for better manipulation and extension
- adjusted readability and documentation
- removed support for non-`sql` backends (for now)
- now works with `qstion` package for better filter handling

- changed checking whether query is paginable - now only requirement is that the ordered column is not present in any nesed expression with `eq` `neq` `in_` or `nin` operator

- now order by columns from `PaginationBuilder` class is able to recognize referenced column and return its referenced name instead of base column name

- added support for `Decimal` type of column in `sql.SQL` backend