impr(result): rework burst heatmap speed brackets (keenemeck)

[keenemeck]

fix for #4678

Indexes values to create uniform distribution, and prevents negative values from existing in burstmap.

Ranges can be modified by changing the decimal values; current distribution is below and appears similar to what is currently used.

<15%, 15%-35%, 35%-65%, 65%-85%, 85%+
where the percentages refer to the amount of words in a given range
i.e. the lowest ~15% of words are in the lowest range, and the next 20% are the in next, etc.

I believe it's less computationally expensive as well, so there's that.

[Miodec]

Thanks