Warn when there are too many missing values to impute

[privefl]

In snp_fastImpute() and snp_fastImputeSimple(), warn when a variant has e.g. more than 20% missing values.
These two functions are designed to impute genotyping arrays with a few missing values, and these variants should have been QCed before, but in the case they have not, this warning could be useful to remind the user.

If someone wants to provide a PR.