Add arg normalizing function for labels and index
Add arg normalizing function for labels and index

Refactored from code in `center_of_mass`, the `_norm_input_labels_index`
function handles the normalization of `input`, `labels`, and `index`
arguments. As these particular arguments will show up repeatedly in the
API, it will be very helpful to have normalize them in the general case
so the API functions can be focused on their particular computations.

dask-image/dask-ndmeasure