Split Unmappable Transactions

[lasuk]

Split transactions that can not be mapped to CashCtrl into multiple transactions that can be mapped to CashCtrl and lead in sum to the same account balance.

This PR introduces a series of transactions that can not be mapped to CashCtrl and then extends CashCtrlClient to split such transactions into collective or multiple transactions that can be mapped. The PR is best reviewed commit by commit.

• 0dc2720 Extend tests to cover FX gain/loss transactions that can not be mapped ATM (expected to fail)
• 0381bc2 Support FX gain/loss transactions (the test introduced above now passes)
• 1d5eba0 Split multi-currency transactions into multiple single-currency transactions
• 7c57683 Ensure integration tests catch approximate equality
• e90655f Extend tests to cover transactions requiring high precision FX rates (expected to fail)
• 82531a9 Support transactions requiring high precision FX rates (the test introduced above now passes)
• f061a45 Avoid unneeded exceptions when transactions requiring high precision FX rates
• 3817155 _get_ledger_attachement does not raise error if file is located in the recycle bin.

[AlexTheWizardL]

I've pushed 2 commits:

• bfad670 Added missing docstrings for the methods and rebased branch onto the main
• 08e0988 Fixes revealed tests failure in this action without setup transitory_account

[lasuk]

Thanks for the docstrings. I have further clarified the description in aecc1731.

Ready for merging.