IMPL: Implement essential DataFrame & Series utility methods

- Add Series helper methods: select_indices, to_f64_vec
- Add DataFrame shape/info: nrow, ncol, shape, dtypes, is_empty, contains
- Add DataFrame row operations: head, tail, slice
- Add DataFrame column operations: select, rename, column_names, select_dtypes
- Add Series statistics: sum, mean, var, sd, min, max
- Add DataFrame statistics: describe, sum, mean
- Refactor extract_series_by_indices into Series::select_indices
- Update module-level documentation with new method signatures and examples

Author: Axect