Issue #246: Transaction does not balance (-1E-24 MR) with @@ (blais/beancount)

[Martin Michlmayr]

New issue 246: Transaction does not balance (-1E-24 MR) with @@
https://bitbucket.org/blais/beancount/issues/246/transaction-does-not-balance-1e-24-mr-with

Martin Michlmayr:

```
2014-01-01 open Assets:Test

2018-03-20 * "Test"
Assets:Test 42.30 USD @@ 5640 MR
Assets:Test -5640 MR
```

gives

```/home/tbm/beancount/test.beancount:9: Transaction does not balance: (-1E-24 MR)
```

Clearly it should balance but it doesn't because of rounding when you convert from a total price to a unit price.

Maybe beancount should automatically infer a tolerance of whatever the precsion of the datatype you use to store is.

Also, another argument for issue #194 to somehow store the total price/cost.