Small stats fixes

[alexhroom]

Fixes #93. There was an erroneous extra count per frame added in #65. Not sure where that came from, but this PR removes it and adds a check to the histogram tests that n is correct.

Also fixes #98 by changing the histograms to use nanoseconds for time ranges internally.

[AnthonyLim23]

the units used in this label don't match those above (nano), I think having it in microseconds is more intuitive, @wilkojohnny what do you think?

[alexhroom]

the display string is in microseconds: see Histogram.__repr__

    pub fn __repr__(&self) -> String {
        let shape = self.hist.shape();
        let mut string = format!(
            "Histogram with:\n  time range {}μs - {}μs",
            (self.min_time.to_micros()),
            (self.max_time.to_micros())

I kept all user interface stuff in microseconds, it just uses nanoseconds internally to avoid unit conversion in the histogram calculation.

[github-actions]

👋 Hi, @alexhroom,

Conflicts have been detected against the base branch. Please rebase your branch against the base branch.

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This message is automatically generated by prince-chrismc/label-merge-conflicts-action so don't hesitate to report issues/improvements there.