Suggestions for return_random_participants

[DrEspresso]

Is your feature request related to a problem? Please describe.
The function does not check whether a centre in centers exists.
The function does not accept percent=1 (maybe useful for consistency checks)
The help page does not explain how the percentage of participants is rounded to integers.

Describe the solution you'd like
A warning should be shown if an element of centers is not an element of ...$ctr

The allowed range of values for percent should include 0 and 1.

The help page should be extended.
There is also one typo:

percantage

[PatrickRWright]

@DrEspresso it would be good to have this implemented by late February just to give a timeline. Can you do that?