Add samples count step as a results output

[grosscol]

Desired State

Emit a document, compatible with mongoimport, with summary statistics from the data processing. Specifically, the number of samples in the variant calling since this cannot be derived from the processed data.

Context

The UI reports the number of genomes from which the data is derived, but this is currently manually coded.
It would be useful for handling multiple data sets if that information could be included as a matter of data processing instead of having to be done manually included. statgen/bravo_vue#4