Class for code that violate cashctrl limitations

[AlexTheWizardL]

Currently cashctrl_ledger class contains "hackish" code that violates cashctrl limitations and allows us to integrate it. Need to create another class and separate the "hackish" code only and leave simple core logic in the cashctrl_ledger class.

To separate "hackish" code while allowing it to be used within core methods, need to use inheritance pattern:

Proposed names for the new Class with "Hackish" Code:

• [x] CashCtrlLedgerExtended

In this example:

• These names indicates that class contains additional, possibly unconventional logic needed for specific integrations.

List of methods that should be moved to the "hackish" class:

• [ ] standardize_ledger()
• [x] sanitize_ledger()
• [x] transitory_account as property
• [x] _add_fx_adjustment()
• [x] split_multi_currency_transactions()

Proposed implementation steps:

1. Define all hackish methods before Lukas vacation.
2. Create a new class that will inherit CashCtrlLedger class.
3. Move hackish methods using inheritance pattern in the new class.
4. Adapt test suits to the new hierarchy.
5. Edit classes docstrings.
6. Update README.md

[lasuk]

Proposal for the class docstring:

class CashCtrlLedger(LedgerEngine):
    """
    Implements the pyledger interface by connecting to the CashCtrl online accounting software.

    See README on https://github.com/macxred/cashctrl_ledger for overview and
    usage examples.
    """