Calculator: improve precision/accuracy trade-off.

This includes multiplication and division in things which should not be
rounded to our determined precision. While this doesn't break any
current tests, it might lead to some surprising floating-point issues.

On the other hand, the other way led to some surprising results where

418.10 / 2 != 418.1 / 2

Hopefully there exists a better long-term solution which we'll find soon
enough.

Addresses issue #509.
Calculator: fix mis-construed style for numbers with initial 0.

There should never be a thousands separator in that place, so don't
allow one.  This was a bit masked by taking all of the numbers in
combination which gave them more context.

Testing a number individually exposed the error.
Merge pull request #510 from mwmiller/calc_precision

Calculator: improve precision/accuracy trade-off.