componentwise `nMarkers()`

[magnusdv]

nMarkers() should have an option to report number of markers for each component.

library(pedtools)

x = list(singleton("A") |> addMarker(),
         singleton("B"))

nMarkers(x)
#> Error: The pedigree components have different number of markers attached

^{Created on 2023-09-06 with reprex v2.0.2}