Choose chart mean rounding precision based on value range

[cstephen]

Closes #65.

This PR addresses overaggressive rounding in charts where we calculate the decadal mean from yearly values client-side. Look at the hovertext in the screenshot below to see what I mean:

Since the mean is rounded to a whole number, 0, it ends up being larger than the max number. And because of this, the plot marker is placed above/outside of the error bars.

To solve this, I've created a new utility function, precisionMean, that calculates the difference between the min/max values and rounds the mean to 1 decimal of precision if the difference is less than 10. If the difference is greater than (or equal to) 10, it continues to round to a whole number.

Here's a screenshot of the same chart using the new precisionMean function:

To test, run the webapp against Zeus:

export RASDAMAN_URL=https://zeus.snap.uaf.edu/rasdaman/ows
export SNAP_API_URL=http://development.earthmaps.io
npm run dev

Then look at the charts for a few of the affected ARDAC items below and make sure the mean rounding in the hovertext is what you'd expect:

http://localhost:3000/item/dd-below-65 http://localhost:3000/item/dd-below-0 http://localhost:3000/item/thawing-index http://localhost:3000/item/freezing-index http://localhost:3000/item/wet-days-per-year http://localhost:3000/item/permafrost-magt http://localhost:3000/item/permafrost-base-top http://localhost:3000/item/permafrost-talik http://localhost:3000/item/indicator-ftc-cmip6 http://localhost:3000/item/indicator-su-cmip6 http://localhost:3000/item/indicator-dw-cmip6 http://localhost:3000/item/indicator-rx1day-cmip6