New features and fixes:
- `DFTable` - clean-up the code
- Created unit tests for `DFTable`
- Implemented the possibility of styling the header in `DFTable`
- Implemented the possibility of styling the index on levels in `DTTable`
- Fixed minor bugs for index showing in `DFTable`
Backwards incompatible changes:
- Moved `Style`, `ColumnStyle`, `RowStyle`, `CellStyle` classes in [qf_lib/documents_utils/document_exporting/element/helpers/style.py](https://github.com/quarkfin/qf-lib/blob/master/qf_lib/documents_utils/document_exporting/element/helpers/style.py)
- Moved `DataType` and `StylingType` in [qf_lib/documents_utils/document_exporting/element/helpers/style_enums.py](https://github.com/quarkfin/qf-lib/blob/master/qf_lib/documents_utils/document_exporting/element/helpers/style_enums.py)