[FEA] add flag to switch between string and numerical type col for group-in-box

[mj3cheun]

Is your feature request related to a problem? Please describe.
string type will result in a histogram listing the x most common cols in graphistry ui
numerical type will result in a histogram showing binned intervals from min to max value in graphistry ui

both can be useful depending on use case, it would be great for a user to be able to switch between the two depending on the situation

see here for how to do the type coersion
cdd1278

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[lmeyerov]

@mj3cheun this feels more like a graphistry thing than a pygraphistry, right?

• in ui, the ability to change the kind of histogram
• in the REST protocol, maybe give hints at this, or maybe make it explicit via arrow supporting something like categorical column types, and we propagate through? see dictionary type @ https://arrow.apache.org/docs/python/data.html