improve parsing of numerical values for v1 JSONs
improve parsing of numerical values for v1 JSONs

This closes #809.

The parsing of numeric values supplied to `prettyString` is improved to both handle large numbers & improve the number of decimal places.

The unused `multiplier` argument to `prettyString` has been removed.

nextstrain/auspice,nextstrain/auspice,nextstrain/auspice