Examples using Multiple Segments

[hunhoffe]

This PR introduces three new programming examples and renames an existing example:

• programming_examples/channel_examples/herd_to_herd was moved to programming_examples/channel_examples/single_segment/herd_to_herd
• A new example, programming_examples/channel_examples/multi_segment/herd_to_herd was added. It does not work.
• To explore the general ability to create multiple segments, I also wrote two examples (that also don't work) that use two segments with herds that don't interact at all.

This issue (#665) was created to note the troubles in using multiple segments.

Part of the goal of writing examples using channels: #648