Jupyter range slider enforcing non-negative integer clims

[tlambert03]

@kephale gave me a nifty usage of ndv with copick: https://gist.github.com/kephale/9cef915fe341ca43b0c46e6b3a45c270

while using it I realized that the clims slider in jupyter (and probably in other widgets) may be truncating and coercing clims to non-negative integers. need to look into it