Change exposure basis minimum to 1000 clients

[jaredlockhart]

There can be cases where the exposure basis population has some large number of clients in the branches but close to 0 but not 0 in the control branch. We set a minimum of 10, but for instance we just saw an example where it was 13 clients and so it defaulted to exposure basis but showed a bunch of spurious results. Let's change the default minimum to 1000.

┆Issue is synchronized with this Jira Task

[mikewilli]

Might be interesting to do this as a percentage of enrolled population.