add error message checking the integer sample size

[LittleBeannie]

Fixes #322

[nanxstats]

Nice... Maybe is.wholenumber() in gsDesign is a useful one-liner internal function to copy here for repeated use:

https://github.com/keaven/gsDesign/blob/bd70f66e346603229b46de2a7a84f0a714857ba1/R/toInteger.R#L159

[LittleBeannie]

Nice... Maybe is.wholenumber() in gsDesign is a useful one-liner internal function to copy here for repeated use:

https://github.com/keaven/gsDesign/blob/bd70f66e346603229b46de2a7a84f0a714857ba1/R/toInteger.R#L159

Thanks for the suggestion. Yeah, gsDesign2 exactly apply the same logic as in gsDesign:::is.wholenumber. Since it is a short function, and we only use it once, shall we just use it within the if condition?

[nanxstats]

I ported and added it as is_wholenumber() to encourage code reuse.