User defined activities

There should be a way of using a C++ defined attribute (or maybe ECL as well!) to create an activity that can generate multiple outputs.

One example would be a splitter that only read a row on one output, another would be a clustering algorithm, with the option to output different results in different formats.

It is heavily related to some other issues, but main differences are
a) The input rows are streamed in,
b) It is executed DISTRIBUTED, with only local rows being read.
c) The outputs are always executed in the same subgraph as the parent activity.