Fix loss of precision in samples.

With smallish numbers (with absolute precision in the tenths place or
less), the sample unit answers would lose precison due to a
misconfigured significant figures printer. Make sure to always print
enough precision. By the way, I don't see any reason not to just crank
the least significant decimal parameter way up.

Test plan:
Enter "5kg m / s^2" as the answer, see "1.12 lb" as a sample solution.

Auditors: alex