Feat: implement precision() an use round_series_to_precision()

[AlexTheWizardL]

This PR removes TODO comments related to rounding by implementing the round_series_to_precision() method.

Key points:

• Replaced hard-coded rounding logic with the newly introduced round_series_to_precision() method.
• Ensured consistent rounding across the codebase, improving precision handling for both individual and vectorized operations.
• Removed all associated TODO comments, as the new method fully addresses the previously deferred rounding logic.
• This PR action will fail until https://github.com/macxred/pyledger/pull/18 is merged

@lasuk Please review the changes and provide feedback or approval for merging

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Flag	Coverage Δ
unittests	93.54% <100.00%> (+0.29%)	:arrow_up:

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Files with missing lines	Coverage Δ
cashctrl_ledger/extended_ledger.py	94.64% <100.00%> (ø)
cashctrl_ledger/ledger.py	93.09% <100.00%> (+0.41%)	:arrow_up:

[lasuk]

Added a few commits to fix errors and simplify the implementation. @AlexTheWizardL: Please shout if you disagree.