clarify dbgAtM issue
look harder for decimal point & digit groups (fixes #196)

Amount display styles have been reworked a bit; they are now calculated
after journal parsing, not during it. This allows the fix for #196:
we now search through the amounts until a decimal point is detected,
instead of just looking at the first one; likewise for digit groups.
Digit groups are now implemented with a better type.

Digit group size detection has been improved a little:
1000,000 now gives group sizes [3,4,4,...], not [3,3,...], and
10,000 gives groups sizes [3,3,...] not [3,2,2,..].
(To get [3,2,2,...] you'd use eg 00,00,000.)

There are still some old (or new ?) issues; I don't think we handle
inconsistent decimal points & digit groups too well. But for now all
tests pass.