A commodity directive that doesn't specify the decimal point character
increases ambiguity and the chance of misparsing numbers, especially
as it overrides all style information inferred from the journal amounts.
In some cases it caused amounts with a decimal point to be parsed as if
with a digit group separator so 1.234 became 1234.
We could augment it with extra info from the journal amounts, when available,
but it would still be possible to be ambiguous, and that won't be obvious.
A commodity directive is what we recommend to nail down the style.
It seems the simple and really only way to do this reliably is to require
an explicit decimal point character. Most folks probably do this already.
Unfortunately, it makes another potential incompatiblity with ledger and
beancount journals. But the error message will be clear and easy to
work around.
