Return the first or last parts of objects inheriting from `mvdf_obj`

# S4 method for mvdf_obj
head(x, n = 6L, ...)

# S4 method for mvdf_obj
tail(x, n = 6L, ...)

Arguments

x	An object inheriting from `mvdf_obj`
n	an integer vector of length up to nrow(mvdf(x)). Values specify the indices to be selected along the rows of the object. A positive value of n[i] includes the first/last n[i] indices in that dimension, while a negative value excludes the last/first abs(n[i]), including all remaining indices. NA or non-specified values (when length(n) < length(dim(x))) select all indices in that dimension. Must contain at least one non-missing value.
...	Ignored.