min sample error catching for --ld

[dvitale199]

Is your feature request related to a problem? Please describe.
when running --ld with <50 samples, the process fails. It is slightly difficult to judge what the issue is in this case

Describe the solution you'd like
up-front checks for sample N with loud warnings for insufficient size for --ld

[dvitale199]

Min sample catch needs to be for founders, specifically. Must have >50 founders for --ld and --het to work