Feat: create memory ledger

[AlexTheWizardL]

This PR covers changes of creating the MemoryLedger class, which extends StandaloneLedger to create a non-persistent ledger system.

This PR is better reviewed commit by commit:

• 71b2dfed81ca7cddcf9dc05409f5b62cb6764bdd - add initial structure of new class
• 3544e729dcc7de68264237665a47eaaae0585a35 - add accessor mutator functions for vat codes and corresponding tests
• 46b0ebf6dc8758a3d7386de067c76ac6adf78b85 - add accessor mutator functions for account charts and corresponding tests
• c6ada998353adba543fb6b5e68e34933df7573f6 - add accessor mutator functions for ledger and corresponding tests
• 2a83e16cf065f54c34bd3531df80a969adce1a2d - covers changes described in #2
• 1ce9608bbd01f91e1cc20de1c8f1d97b56c47ee7 - add mirroring logic for VAT, accounts, and ledger with corresponding tests

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.22989% with 17 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pyledger/memory_ledger.py	91.20%	8 Missing :warning:
pyledger/ledger_engine.py	89.70%	7 Missing :warning:
pyledger/standalone_ledger.py	81.81%	2 Missing :warning:

Flag	Coverage Δ
unittests	64.62% <90.22%> (+6.20%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pyledger/__init__.py	100.00% <100.00%> (ø)
pyledger/constants.py	100.00% <100.00%> (ø)
pyledger/test_ledger.py	100.00% <100.00%> (ø)
pyledger/standalone_ledger.py	69.64% <81.81%> (+0.55%)	:arrow_up:
pyledger/ledger_engine.py	65.43% <89.70%> (+6.01%)	:arrow_up:
pyledger/memory_ledger.py	91.20% <91.20%> (ø)

[lasuk]

Thanks for the final modifications. Ready to merge ...

It's a pleasure to see our concept implemented. Quite convincing!