Add First Version of Integration Tests

[lasuk]

Add a test to ensure that account balances are correctly calculated for a complete ledger system with tax and currency revaluations.

This first version of integration test is limited in scope, but provides sufficient coverage to ensure that the upcoming refactoring of the StandaloneLedger class does not break the functionality.

• Only tests the memory ledger class. Tests can be easily extended to other classes ones accessors and mutators for price and revaluations are implemented.
• Limited number of test cases: ATM tax codes exclusive of VAT or with a contra account are not tested.

My goal was to quickly set a solid foundation for the upcoming refactoring step. Test coverage can be extended any time, and the test may be relocated to a different unit in a later version.

[codecov-commenter]

Codecov Report

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

Flag	Coverage Δ
unittests	82.99% <100.00%> (+13.54%)	:arrow_up:

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Files with missing lines	Coverage Δ
pyledger/ledger_engine.py	74.49% <100.00%> (+14.65%)	:arrow_up:
pyledger/standalone_ledger.py	77.64% <100.00%> (+38.21%)	:arrow_up:

... and 1 file with indirect coverage changes