Document default seeding

[NeuroShepherd]

The current default random seeding approach is to set the seed for each run as equal to the run number, e.g. run number 5 includes set.seed(5). This should be clearly documented in at least two places IMO:

1. In a vignette
2. Directly in the app, possibly as a modal or hover textbox

[NeuroShepherd]

See section 4 of this article for discussion on seeding https://onlinelibrary.wiley.com/doi/10.1002/sim.8086

R uses Mersenne-Twister as default which some people consider to be good enough, and some call for a replacement method https://arxiv.org/pdf/1910.06437.pdf

[NeuroShepherd]

See also https://cran.r-project.org/web/packages/dqrng/vignettes/dqrng.html for xoroshiro RNG implementation