Prohibit categorical functions with fractional stop domain values

[jfirebaugh]

Because categorical functions rely on equality-based identity, and floating point values cannot reliably be compared for equality, the validator should prohibit categorical functions which use stop domain values that aren't integers. For example, the following should be disallowed:

{
  "type": "categorical",
  "property": "ratio",
  "stops": [
    [0.333, 0.3],
    [0.666, 0.6]
  ]
}