nMarkers() by default returns a single number; the number of marker
objects attached to x. If x is a ped list, an error is raised if the
components have different numbers of markers. This check can be skipped by
setting compwise = TRUE, in which case the function returns a vector of
the component-wise marker numbers.
The function hasMarkers(x) returns TRUE if (at least component of) x
has attached markers, otherwise FALSE. If compwise = TRUE, a logical
vector of the same length as x.