Fix invalid array syntax in json schema

[jenshnielsen]

The current format is invalid. It should either be just a single item type or like this pr suggest a list of item types which defines exactly the type of each element

Since cutoff_percentile always has two numbers this seems like the correct format.

See https://json-schema.org/understanding-json-schema/reference/array.html for some details

This also adds a pre-commit hook that check that json is valid which would have caught this. I had to exclude asv.conf.json since it contains comments which is not valid json

[codecov]

Codecov Report

Merging #3097 (a77a421) into master (3114b59) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3097   +/-   ##
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  Coverage   65.65%   65.65%           
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  Files         216      216           
  Lines       28746    28746           
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  Hits        18874    18874           
  Misses       9872     9872           

[astafan8]

great catch!