Add center value for diverging map brackets

[paarriagadap]

Core problem

When using a diverging scale for a map, Grapher automatically defines a center value (where colors change) which doesn't necessarily reflect how the measure works.

Proposed solution

Adding a "center value" field into Grapher can fix this, expressing exactly where we want this transition point to happen.

Alternatives

The alternative, for now, is creating a symmetric range where the midpoint is exactly this transition point, but that generates brackets that do not cover the values as we might like.

Context

See original post in Slack. In this example, I would like the colors to start changing (from red to blue) from 0 and not from 4:

The alternatives to fix this would be to create symmetrical brackets, as ... which is not ideal.

There are tools like Tableau that include this kind of functionality:

[sophiamersmann]

Zero seems to be commonly used as the center value, but there are also other cases where centering around 1 or 50 makes sense (as mentioned by Ed on Slack):

• https://ourworldindata.org/grapher/male-female-ratio-suicides-rates?tab=map
• https://ourworldindata.org/grapher/share-population-female

[danyx23]

Since #3488 requests another version of this feature we are bumping this one up in priority