Compute slice indices from slice definition.
get.slice.indices(voldim, axis, slices)integer vector, the dimension of the volume
integer, the axis
slice index definition. If a vector of integers, interpreted as slice indices. If a single negative interger `-n`, interpreted as every `nth` slice, starting at slice 1. The character string 'all' or the value `NULL` will be interpreted as *all slices*.
integer vector, the computed slice indices. They are guaranteed to be valid indices into the volume.