Improve in-code documentation of `backend.dynapcnn.mapping`

[bauerfe]

The module sinabs.backend.dynapcnn.mapping is used to find a mapping from the layers in a DynapcnnNetwork to layers on a device, under consideration of the chip layers' memory constraints.

This is done by formulating the problem as a bipartite matching problem and to solving it with the Edmonds-Karp Algorithm.

In-code documentation of this module is rather sparse, so that it's hard for new developers to understand what is going on.