Sampling ui

[okennedy]

This PR implements sampling (resolves VizierDB/web-ui#34)

It relies on the following PR on Mimir UBOdin/mimir#368

[julianafreire]

In the design, is it possible to have multiple sampling strategies? I ask because in some of the machine learning use cases we have worked with, we must use stratified sampling. And if we are cleaning for machine learning, this will be needed.

[okennedy]

@julianafreire : This PR adds cell types for the following three sampling modes:

1. Naive sampling (a flat x% of the data)
2. Manual stratified sampling (bin records by a column, provide an explicit % rate for each bin)
3. Automatic stratified sampling (like 2, but pick % rates automatically to ensure equal representation from each bin)

[okennedy]

@julianafreire : See https://github.com/VizierDB/web-ui/wiki/Cell-Sampling for more details.