feat: add FX_revaluation method and corresponding tests

[AlexTheWizardL]

This PR covers creating the FX_revaluation() method that triggers CashCtrl to automatically allocate foreign exchange gains or losses based on the provided exchange rates. This logic is covered with a test suit.

@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	94.43% <100.00%> (+0.40%)	:arrow_up:

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Files	Coverage Δ
cashctrl_ledger/constants.py	100.00% <100.00%> (ø)
cashctrl_ledger/ledger.py	93.77% <100.00%> (+0.47%)	:arrow_up:

[lasuk]

Thanks, @AlexTheWizardL, for this PR, and for your flexibility in making it a priority!

I've added two commits. Please yell if you disagree:

• a24c60e9: Getter and setter for fx_gain_loss_account
• a93a2848: Concise and to the point docstring, clarify argument names and code

The provided functionality already covers my use case. I have three suggestions for further improvement:

• Use enforce_dtypes to ensure the data frame has required columns of the right type.
• Make the exchange_rate an optional column, allow for NAs. If NA, the system's default exchange rate rate for the given date is used.
• Make 'accounts' optional (can be None). If not provided, use all foreign currency accounts covered by GET EXCHANGEDIFF, the default exchange rate highlighted above, and the default FX gain/loss account in the settings.

What do you think?

[AlexTheWizardL]

I`ve added 3 commits to covers your proposed improvements:

• Use enforce_dtypes to ensure the data frame has required columns of the right type. 5244cb8
• Make the exchange_rate an optional column, allow for NAs. cd23ad0
• Make 'accounts' optional (can be None). c2defa9

Please yell if you disagree.