schwab_equity_award_json.py parser: avoid unnecessary quantity recalculation.

[m01]

This can introduce rounding errors.

This fixes #574.

Basically, there's some code in cgt_calc/parsers/schwab_equity_award_json.py, which tries to work around some issues in data exported from Schwab:

            # Schwab's data export sometimes lacks decimals on Sales
            # quantities, in which case we infer it from number of shares in
            # sub-transactions, or failing that from the amount and salePrice.

This codepath is triggered unconditionally when an integer number of shares is sold - even when there isn't an issue with the exported data. This change makes that codepath conditional on there actually being an issue with the exported data.

Without the change in cgt_calc/parsers/schwab_equity_award_json.py, the test triggers the following error:

FAILED tests/test_schwab_equity_award_json.py::test_schwab_transaction_v2_rounding - AssertionError: assert Decimal('91.00001463507441935342241215') == Decimal('91')

The test data is fictional and purely there to reproduce the error, but it is plausible based on data I've seen and I have seen the rounding errors happen in real data.

Note that because quantities matter for S104 pool calculations this may introduce deltas in outputs between
calculations before and after this change. However, without this change you may run into asserts, depending
on what kinds of trades/gifts you make.

This contribution has been developed in my spare time. Disclaimer: I'm not a financial advisor/accountant.

[KapJI]

Looks good to me, thanks for adding tests!