add single cell example data

[ssnn]

While improving the vignettes for distToNearest, @Julian Zhou noticed we need to add a single cell example dataset that can be used to better illustrate how to use distToNearest. Currently, the code chunk has eval='FALSE'( see here). We need to add a single cell example dataset that can also be used in the 10x tutorial. @Kenneth Hoehn and @Roy Jiang will identify suitable dataset. @Hailong Meng can help storing data in clip.

Assigning to Ken because this issue is related to immcantation/issues/47.