[fix] range slider doesn't work when step < 1 in dataset filter

[igorDykhta]

• In the dataset filter, the value range could be small [-122.4548828, -122.4545319], which is common when e.g. filtering by latitude or longitude column. In this case, the range slider doesn't work correctly. It is caused by the normalizeSliderValue() function in range-brush.js, which works when step >= 1. When step < 1, the normalizeSliderValue() always returns a rounded value (or mark).