This function checks whether v is a vector or factor with
length equal to npoints (or length equal to 1 if
oneok=TRUE), not containing any NA values (unless
naok=TRUE).
If these requirements are all satisfied, the result is the logical
value TRUE.
If not, then if fatal=TRUE (the default), an error occurs;
if fatal=FALSE, the result is the logical value FALSE
with an attribute describing the requirement that was not satisfied.