code: public static <T> WindowOver<T> nthValue(Expression<T> expr, Number n) {
          return nthValue(expr, ConstantImpl.create(n));
      }
docs: NTH_VALUE returns the expr value of the nth row in the window defined by the analytic clause.
  The returned value has the data type of the expr.
  
  @param expr measure expression
  @param n one based row index
  @return nth_value(expr, n)