Add `.seek(...)` to streams

[SSoelvsten]

In all algorithms we have something akin to the following from internal/quantify.h

while (v.uid < t_seek) {
  v = in_nodes.pull();
}

We may as well just add a seek function to node streams, that return the very value of this while-loop.

v = in_nodes.seek(t_seek);

Tasks (done in #397)

• [x] Add .seek(...) function to file_stream
• [x] Add .seek(...) function to node_stream.

Algorithms to Update

• [x] count
• [ ] pred
• [ ] intercut
• [ ] prod2
• [ ] prod3
• [ ] quantify
• [x] select