ENH/COMP: provide graphConstRow accessor

- the previous code used a dual-purposed graphRow for non-const/const
  access. However, clang 4.0 rightly identifies there being no
  constructor path from "graphRow<T>" to a "graphRow<const T>".

  Since the row-access is a somewhat similar concept to an STL-iterator,
  now provide non-const and const versions of the row access.