- uniq() : creates an IndirectList with duplicated entries
  filtered out

- subset() : creates an IndirectList with positions that satisfy
  a condition predicate.

- subset_if() : creates an IndirectList with values that satisfy a
  given predicate.

  An indirect subset will be cheaper than creating a subset copy
  of the original data, and also allows modification.

STYLE: combine UIndirectList.H into UIndirectList.H (reduce file clutter)