Scaling factor should support fractional values

I see that from #19 the decision was made to match the Perl implementation by truncating integers. This was perhaps in part due to an "accumulation of error" from [#18](https://github.com/brendangregg/FlameGraph/pull/18)

But, the [documentation](https://docs.rs/inferno/0.11.3/inferno/flamegraph/struct.Options.html#structfield.factor) specifies that

> For example, if you have 23.4 as a sample count you can upscale it to 234, then set factor to 0.1

Which implies that decimal points are supported.

In [firestorm](https://crates.io/crates/firestorm) the units are in nanoseconds. At least on my computer, the minimum resolution for a sample is 100ns. To "unclutter" the text, I would like to display microseconds instead with fractional values. Eg: instead of 17,100ns, show 17.1μs. This would require fractional values to do well. The current behavior sometimes ends up truncating values to 0.

The accumulation of error can be avoided by not using floating point for calculations throughout, but instead only use the `factor` at the last moment when formatting the SVG text.

Start with inferno::flamegraph::Options.factor and trace how the scaling factor reaches SVG text formatting. Compare the documented fractional example with the current truncation behavior and the firestorm nanosecond use case. Done means fractional factors produce nonzero decimal display values without accumulating calculation error.

Written by the indexing model from the issue text.