Datasiphon

- substitutions for reconstructing filter are now in class and are accepted as list of `Substitution` objects
- unused substitutions are now added to filters

- added option to `PaginationBuilder` to provide `removals` as for removing fields from filter

- removed operations as methods from `QueryBuilder` class
and rewritten them as separate classes for more flexibility and easier extension
- implemented `Paginationbuilder` class for rebuilding filter from query providing filter which can be adjusted

- added helper method in `sql.SQL` backend - `is_query_paginable` - to check if query is paginable (for pointer based pagination) - based on selected columns

- Added support for parsing `datetime` and `date` objects in `sql.SQL` backend from `str` format (assuming iso-format of string)
- New class `FilterTypeParser` added to `sql.SQL` backend to return constructor for given type (e.g. `datetime`, `date`, `int`, `float`, `str`, `bool`) - now able to parse `datetime` and `date` objects from `str` format

- Validation changes: now instead of checking whether value in filter is of instance `column.type.python_type`, it uses the given `python_type` constructor to check if the value is processable by the given type
- Operations now apply the `python_type` constructor to the value before applying the operation