Small development changes (see full message below).

- In function `_cols_operation_balance_by_instrument_for_group` changed `prev_operation_balance[<colname>]` for `df.loc[prev_idx, <colname>]` as this is easier to understand, it shows that we are accessing the previous index value.
- Implemented the usage of `with pd.option_context('future.no_silent_downcasting', True):` for `.fillna()` to avoid unexpected downcasting. See pandas-dev/pandas#57734 (comment) . Used throughout `cols_operation*` functions.
- Removed usage of `DataFrame.convert_dtypes()` as it doesn't simplify dtypes, it only passes to a dtype that supports pd.NA. See pandas-dev/pandas#58543 .
- Added `DataFrame.infer_objects()` when returning the ledger or `cols_operation*` functions to try to avoid objects if possible.
- Changed the structure for `cols_operation*` functions:
    - Added a verification of `self._ledger_df`, if empty the function returns an empty DataFrame with the structure needed. Allows for less computing if empty.
    - The way the parameter `show_instr_accnt` creates a return with columns ['instrument', 'account'] is structured the same way on all functions.
- Simplified how the empty ledger is created in `_create_empty_ledger_df`.
- Changes column name 'balance sell profit loss' to 'accumulated sell profit loss'.
- Minor code fixes.
- Minor formatting fixes.

Author: caballerofelipe