Deprecate DataFrameMixin

The `DataFrameMixin` was originally introduced in the `space` module to share functionalities between `GridPandas` and `GridPolars`. Since the pandas implementation has now been deprecated, this mixin is no longer necessary.

Deprecating the `DataFrameMixin` will simplify the codebase and make the Polars implementation cleaner. Additionally, it will make future refactoring and the transition to Polars `LazyFrames` easier.

**Tasks:**
- [ ] Identify and remove all usages of `DataFrameMixin` within the `space` module.
- [ ] Refactor the Polars-related implementations accordingly.
- [ ] Ensure existing functionality remains intact by updating unit tests.

Start by locating all DataFrameMixin usages in the space module, then inspect the Polars-related implementations and their unit tests. Refactor those implementations to remove the mixin while preserving existing functionality, and update or run the unit tests to verify the result.

Written by the indexing model from the issue text.